Add "al_get_render_state"

[MarkOates]

During some debugging, I noticed I didn't have access to the current render state, which would be helpful. This would be particularly useful in instances where different parts of a game system manage render state differently, and checking for possible leaky render state. The values for it seem to be stored in ALLEGRO_DISPLAY.

Some points to be aware of in this PR:

• In the refman/, not sure what "Since 5.2.11" to put it under.
• Not sure about a return value or behavior when there is no current display. This implementation just has it return 0 with no warning or anything.

[SiegeLord]

Thanks!

[MarkOates]

Sweet! I missed the merge notification on this one.

I've already had several times where this would be useful. Thank you!