Support double values that start with a dot for implicit commands

src/Avalonia.Visuals/Media/PathMarkupParser.cs

@@ -424,7 +424,7 @@ namespace Avalonia.Media
         {
             span = SkipWhitespace(span);
 
-            return !span.IsEmpty && (span[0] == ',' || span[0] == '-' || char.IsDigit(span[0]));
+            return !span.IsEmpty && (span[0] == ',' || span[0] == '-' || span[0] == '.' || char.IsDigit(span[0]));
         }
 
         private static bool ReadArgument(ref ReadOnlySpan<char> remaining, out ReadOnlySpan<char> argument)

tests/Avalonia.Visuals.UnitTests/Media/PathMarkupParserTests.cs

@@ -162,6 +162,7 @@ namespace Avalonia.Visuals.UnitTests.Media
         }
 
         [Theory]
+        [InlineData("M5.5.5 5.5.5 5.5.5")]
         [InlineData("F1M9.0771,11C9.1161,10.701,9.1801,10.352,9.3031,10L9.0001,10 9.0001,6.166 3.0001,9.767 3.0001,10 "
                     + "9.99999999997669E-05,10 9.99999999997669E-05,0 3.0001,0 3.0001,0.234 9.0001,3.834 9.0001,0 "
                     + "12.0001,0 12.0001,8.062C12.1861,8.043 12.3821,8.031 12.5941,8.031 15.3481,8.031 15.7961,9.826 "