An example of what could be the equivalent of
"RBG = REPLACE(TEXTURE)
A = MODULATE(PREVIOUS,TEXTURE)"
using the ARB_texture_env_combine extension was given, but it seems that
a few typo were left:
* remove a spurius GL_COMBINE_ALPHA
* use the _ALPHA variant of SRCN and OPERANDN when setting up the
alpha combiner
87d19b8d18