To apply a mask to a RGB (non-paletted) cursor in SurfaceSdl, we use an
intermediary masked_image, where the pixels made transparent by the mask
are replaced by an ARGB color with alpha = 0 (ARGB 0x00000000 in practice).
However the cursor surface still has a key color attached to it by
SDL_SetColorKey (set to 0 currently), which is not sensitive to the alpha
channel, wrongly making opaque black ARGB 0xFF000000 transparent as well.
So we provide an option to disable the key color in such a case.
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