* define OPENGL, LOADGL, and TREMOR macros in the makefile
* use the above macros in the code, making it easier to read, maintain, and port
* some general cleanup of makefile
* enable GPU-accelerated brightness/gamma correction in OpenGL
* OpenGL: do brightness correction with the fixed-function pipeline; this is a better way to do it even for GPUs that support GLSL. However, GLSL is still used for both brightness and gamma when gamma correction is requested.
* OpenGL: remove dead/disabled code
* dynamically load OpenGL libraries on OS X
