bitmapinfoheader: don't read/write the reserved RGBQUAD field in palette

The palette is appended as RGBQUAD after the BITMAPINFO [1] [2]. The RGBQUAD 4th value is reserved and should be 0 [3].

So we write it as 0 and read it as 0xFF (fully opaque RGBA). If the palette had alpha information it will be lost on writing.

[1] https://learn.microsoft.com/en-us/windows/win32/api/wingdi/ns-wingdi-bitmapinfoheader [2] https://learn.microsoft.com/en-us/windows/win32/api/wingdi/ns-wingdi-bitmapinfo [3] https://learn.microsoft.com/en-us/windows/win32/api/wingdi/ns-wingdi-rgbquad