koptocr: fix and optimize bitmap2pix
implementation
#60
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The previous implementation would only work when converting the whole source bitmap (x = 0, y = 0, same source & destination width / height).
Additionally, avoid one unnecessary temporary conversion to a greyscale bitmap when source is 24bit.
Example of borked conversion (
24bpp 461×628 → +0+0:460×627
):This change is