[php-src] Issue #11366: Add support for other bit depth for imagecreatefrombmp

Issue: https://github.com/php/php-src/issues/11366 Author: hoit ### Description Hello, In my application, i need to load different kind of bitmaps to apply some checks on it. I see that imagecreatefrombmp works correctly for bitmap file which have indexed color with 8 bit depth only. The function return false if the image is : - not in indexed color mode - in index color mode with 4 bpp (16 colors). I saw that some libs are able to manage it or found some pieces of code to do it but it would be great to add it natively ! Is it possible ? Thanks in advance