Added the ability to change the screenshot format with a new CVar:
r_screenshotFormat. This has four options: 0 (TGA), 1 (BMP), 2 (PNG),
and 3 (JPG). JPG quality can be selected with a new CVar:
r_screenshotJpgQuality. TGA is the default to preserve original behavior
from DOOM 3.
A new dependency on stb_image_write has been added. It has been
included. No additional configuration with CMake is required.
