BMP enc: fix the transparency case
BMP enc: fix the transparency case

When transparency is present, it's not enough to just write 32bpp samples.
One need to use the full BITMAPV3INFOHEADER syntax and specify the
masks for BGRA.

 see https://en.wikipedia.org/wiki/BMP_file_format#Pixel_storage

Also remove the height-flip trick and write samples bottom-to-top instead.

Change-Id: If5d92c11453b96764b5bfbf19e9678e632bc911f

webmproject/libwebp,webmproject/libwebp,webmproject/libwebp,webmproject/libwebp,webmproject/libwebp