Add al_save_bitmap_flags

[SiegeLord]

This is specifically to enable undoing pre-multiplied alpha.

[RogerLevy]

Discovered the quirk while working on my paint program. Not hard to implement within my app but there should be an official way.

[RogerLevy]

I'd add, it'd probably be appropriate to add native support for higher-resolution formats like 16-bit PNG or float OpenEXR for this to be practical. Otherwise wouldn't the RGB precision of translucent pixels degrade over time?

[SiegeLord]

Sure, but if you're concerned about that, it's probably best to use non-premultiplied alpha throughout the program. Premultiplied alpha can be disabled at loading time via al_load_bitmap_flags, and at drawing time by changing the blender to al_set_blender(ALLEGRO_ADD, ALLEGRO_ALPHA, ALLEGRO_INVERSE_ALPHA)

[SiegeLord]

This isn't necessarily so drastic a suggestion, as for an image editing program it probably makes sense to operate in the 'raw', unadulterated format.