Bitmap export is pixel conversion, not an ImmutableBitmap-specific
operation. It needs a source bitmap and color space, and callers should
provide those explicitly instead of routing through the immutable
snapshot abstraction.
Move the export formats, flags, result type, and conversion
implementation into a new BitmapExport helper. Keep
BitmapExportResult.h as a forwarding header for existing includes
while making BitmapExport.h the new home for the public API.
Update WebGL and the LibGfx export test to use the standalone helper
directly.
