Common: simplified few Bitmap member functions

This also corrects Bitmap::CreateTransparent() behavior which caused problem when called with color_depth = 0.
This fixes crashing inventory window called by old InventoryScreen() script function.
Merge branch 'master' into develop-3.3.1
Engine: tidying main loop a bit

* renamed several functions to better reflect their meaning;
* regrouped some code;
* removed unused 'want_quit' variable
Engine: refactoring dialog options routine

Split show_dialog_options() into parts. Got rid of "goto" in the loop, implemented two distinct functions for drawing and updating instead.
Engine: refactoring old inventory screen routine

Split __actual_invscreen() into parts. Got rid of "goto" in the loop, implemented two distinct functions for drawing and updating instead.