The version string for ES 1 is "OpenGL ES-CM 1.x" or
"OpenGL ES-CL 1.x". For ES 2 it is "OpenGL ES 2.0", while it
seems to be only "3.x" for ES 3.

Since the code is mostly built for either ES 1 or ES 2 (only
Apple support targeting both at the same time, from the same
built object code - but we only use ES 2 on iOS), simplify
this down to a compile time decision.
Signed-off-by: Martin Storsjö <martin@martin.st>