[SDL] 2D API evolution (SDL 1.3/2.0)

[Sam Lantinga]

> What is the difference between color blend,
> const. alpha source modulation, blend operation ?

Here are the combinations:
dst = src
dst = (src * modulation)
dst = src BLEND dst
dst = (src * modulation) BLEND dst

This allows tinting and constant alpha in combination with RGB and RGBA blits.

	-Sam Lantinga, Senior Software Engineer, Blizzard Entertainment