BufferSpan ref indexer + Fix Drawing usages

src/ImageSharp.Drawing/Brushes/ImageBrush{TColor}.cs

@@ -119,7 +119,7 @@ namespace ImageSharp.Drawing.Brushes
 
                 using (PinnedBuffer<float> buffer = new PinnedBuffer<float>(scanlineBuffer))
                 {
-                    BufferPointer<float> slice = buffer.Slice(offset);
+                    BufferSpan<float> slice = buffer.Slice(offset);
 
                     for (int xPos = 0; xPos < scanlineWidth; xPos++)
                     {

src/ImageSharp.Drawing/Brushes/PatternBrush{TColor}.cs

@@ -152,7 +152,7 @@ namespace ImageSharp.Drawing.Brushes
 
                 using (PinnedBuffer<float> buffer = new PinnedBuffer<float>(scanlineBuffer))
                 {
-                    BufferPointer<float> slice = buffer.Slice(offset);
+                    BufferSpan<float> slice = buffer.Slice(offset);
 
                     for (int xPos = 0; xPos < scanlineWidth; xPos++)
                     {

src/ImageSharp.Drawing/Brushes/Processors/BrushApplicator.cs

@@ -57,7 +57,7 @@ namespace ImageSharp.Drawing.Processors
 
             using (PinnedBuffer<float> buffer = new PinnedBuffer<float>(scanlineBuffer))
             {
-                BufferPointer<float> slice = buffer.Slice(offset);
+                BufferSpan<float> slice = buffer.Slice(offset);
 
                 for (int xPos = 0; xPos < scanlineWidth; xPos++)
                 {

src/ImageSharp.Drawing/Brushes/RecolorBrush{TColor}.cs

@@ -143,7 +143,7 @@ namespace ImageSharp.Drawing.Brushes
 
                 using (PinnedBuffer<float> buffer = new PinnedBuffer<float>(scanlineBuffer))
                 {
-                    BufferPointer<float> slice = buffer.Slice(offset);
+                    BufferSpan<float> slice = buffer.Slice(offset);
 
                     for (int xPos = 0; xPos < scanlineWidth; xPos++)
                     {

src/ImageSharp.Drawing/Brushes/SolidBrush{TColor}.cs

@@ -91,7 +91,7 @@ namespace ImageSharp.Drawing.Brushes
 
                 using (PinnedBuffer<float> buffer = new PinnedBuffer<float>(scanlineBuffer))
                 {
-                    BufferPointer<float> slice = buffer.Slice(offset);
+                    BufferSpan<float> slice = buffer.Slice(offset);
 
                     for (int xPos = 0; xPos < scanlineWidth; xPos++)
                     {

src/ImageSharp/Common/Memory/BufferSpan{T}.cs

@@ -106,6 +106,23 @@ namespace ImageSharp
         /// </summary>
         public IntPtr PointerAtOffset { get; private set; }
 
+        /// <summary>
+        /// Returns a reference to specified element of the span.
+        /// </summary>
+        /// <param name="index">The index</param>
+        /// <returns>The reference to the specified element</returns>
+        public ref T this[int index]
+        {
+            [MethodImpl(MethodImplOptions.AggressiveInlining)]
+            get
+            {
+                DebugGuard.MustBeLessThan(index, this.Length, nameof(index));
+
+                byte* ptr = (byte*)this.PointerAtOffset + BufferSpan.SizeOf<T>(index);
+                return ref Unsafe.AsRef<T>(ptr);
+            }
+        }
+
         /// <summary>
         /// Convertes <see cref="BufferSpan{T}"/> instance to a raw 'void*' pointer
         /// </summary>

tests/ImageSharp.Tests/Common/BufferSpanTests.cs

@@ -209,6 +209,51 @@ namespace ImageSharp.Tests.Common
         }
 
 
+        public class Indexer
+        {
+            public static readonly TheoryData<int, int, int> IndexerData =
+                new TheoryData<int, int, int>()
+                    {
+                        { 10, 0, 0 },
+                        { 10, 2, 0 },
+                        { 16, 0, 3 },
+                        { 16, 2, 3 },
+                        { 10, 0, 9 },
+                        { 10, 1, 8 }
+                    };
+
+            [Theory]
+            [MemberData(nameof(IndexerData))]
+            public void Read(int length, int start, int index)
+            {
+                Foo[] a = Foo.CreateArray(length);
+                fixed (Foo* p = a)
+                {
+                    BufferSpan<Foo> span = new BufferSpan<Foo>(a, p, start);
+
+                    Foo element = span[index];
+
+                    Assert.Equal(a[start + index], element);
+                }
+            }
+
+            [Theory]
+            [MemberData(nameof(IndexerData))]
+            public void Write(int length, int start, int index)
+            {
+                Foo[] a = Foo.CreateArray(length);
+                fixed (Foo* p = a)
+                {
+                    BufferSpan<Foo> span = new BufferSpan<Foo>(a, p, start);
+
+                    span[index] = new Foo(666, 666);
+
+                    Assert.Equal(new Foo(666, 666), a[start + index]);
+                }
+            }
+        }
+
+
         public class Copy
         {
             private static void AssertNotDefault<T>(T[] data, int idx)
@@ -443,6 +488,7 @@ namespace ImageSharp.Tests.Common
                 }
             }
 
+
             internal static bool ElementsAreEqual(Foo[] array, byte[] rawArray, int index)
             {
                 fixed (Foo* pArray = array)