Works great on Mac now. Will test on Windows & Linux (Mesa) tomorrow. Related to T49505
Main fix is glActiveTexture and immUniform1i.
TEXTURE_2D vs TEXTURE_RECTANGLE is now a compile-time option. Both are available starting in GL 3.1 so there's no need for a run-time check.
Removed glClears that I don't think are necessary.
Prevent TEXTURE_2D from creating extra mipmap levels. We only need level 0.
Some minor cleanup: booleans and variable declarations.