Refactor shader handling

This fixes a few of the bad design descisions of previous shader
iterations. This uses a dedicated class for managing the resources and
uniform locations of a shader program.

Uniforms are also stored in this object which makes a lot more sense
than storing them in the OpenGL state where they had to be reset every
time a new shader was used.

There are still a lot of possible enhancments to make, e.g. by using
uniform blocks and buffers instead of standard uniforms which would make
uniform location handling easier.

Another optimization that I will investigate is reducing the amount or
different compiled shaders by turning some of the current compile-time
defines into bool uniforms.

code/parse/lua.cpp

@@ -12,7 +12,7 @@
 #include "graphics/2d.h"
 #include "graphics/font.h"
 #include "graphics/generic.h"
-#include "graphics/gropenglpostprocessing.h"
+#include "graphics/opengl/gropenglpostprocessing.h"
 #include "headtracking/headtracking.h"
 #include "hud/hudbrackets.h"
 #include "hud/hudconfig.h"