BUFFER_WIDTH (float) : The screen width
BUFFER_HEIGHT (float) : The screen width

EFX : So the shaders know what injector we are using
OPENGL
DIRECT3D9
DIRECT3D10
DIRECT3D11) : So the shaders know what API we are using

EFX_HLSL_2_0
EFX_HLSL_2_1
EFX_HLSL_3
EFX_HLSL_4
EFX_HLSL_4_1
EFX_HLSL_5
EFX_GLSL_1 ... EFX_GLSL_4