Print a warning message when using set_mode(x,y,8|16|32)

[Pablo Antonio Navarro Reyes]

It would help new PixTudio's users to avoid using the deprecated bpp parameter.

Luckily, those numbers won't collide with the render flags (which start from 256), so it's not that important at the moment.

[Joseba Echevarria García]

• Removed the M640x480 (and simmilar constants).
• Removed the variation of set_mode() which accepts one of the removed constants
• Added a deprecation warning to set_mode() that will be shown if the user tries to set the bit depth.

Closes Bug #20

→ <<cset 6008e8f619ca>>