The PNG decoder, when dealing when indexed images with a palette, could only decode a bit depth of 8. Teach it to decode depths of 1, 2 and 4 as well. As the palette data is exposed to consumers of the PNG class, unpack the data into a 8 bit depth so consumers don't need to also handle the new bit depths.
This rule no longer appears to be buggy, so enforce it. Some of the automated fixes are adjusted in order to improve the result. #pragma directives have no option to control indentation, so remove them where possible.
Also avoid ReadBytes calls that allocate a buffer by either updating the stream position (if not interested in the bytes), by reusing an input buffer (if interested in the bytes), or using a stackalloc buffer to avoid the allocation (for small reads).
Our SpriteFrameType names refer to the byte channel order rather than
the bit order, meaning that SpriteFrameType.BGRA corresponds to the
standard Color.ToArgb() etc byte order when the (little-endian) integer
is read as 4 individual bytes.
The previous code did not account for the fact that non-indexed Png
uses big-endian storage for its RGBA colours, and that SheetBuilder
had the color channels incorrectly swapped to match and cancel this out.
New SpriteFrameType enums are introduced to distinguish between BGRA
(little-endian) and RGBA (big-endian) formats, and also for 24bit data
without alpha. The channel swizzling / alpha creation is now handled
when copying into the texture atlas, removing the need for non-png
ISpriteLoader implementations to allocate an additional temporary array
and reorder the channels during load.
