* decoding bitmaps with Bitfields masks
* added testcases for Bitfields bitmaps
* added parsing of the complete bitmap V4 header to get the color masks infos for the BITFIELDS compression
* writing now explicitly a Bitamp v3 header (40 bytes)
* added test image for issue #735
* fixed rescaling 5-bit / 6-bit to 0 - 255 range
* BitmapEncoder now writes BMP v4 header
* using MemoryMarshal.Cast to simplify parsing v4 header
* added testcases for bitmap v3, v4, v5
* Bitmap encoder writes again V3 header instead of V4. Additional fields for V4 are zero anyway.
* added parsing of special case for 56 bytes headers
* using the alpha mask in ReadRgb32BitFields() when its present
* added support for bitmasks greater then 8 bits per channel
* using MagickReferenceDecoder reference decoder for the test with 10 bits pixel masks
* changed bitmap constants to hex
* added enum for the bitmap info header type
* added support for 52 bytes header (same as 56 bytes without the alpha mask)
* clarified comment with difference between imagesharp and magick decoder for Rgba321010102
* BmpEncoder now writes a V4 info header and uses BITFIELDS compression
* workaround for issue that the decoder does not decode the alpha channel correctly: For V3 bitmaps, the alpha channel will be ignored during encoding
* Fix#732
Brian Popow
James Jackson-South