gles2-context: only insert prelude + wrapper into strv once
gles2-context: only insert prelude + wrapper into strv once

In our wrapper for glShaderSource we special case when a vertex shader
is being specified so we can sneak in a wrapper for the main function to
potentially flip all rendering upside down for better integration with
Cogl.

Previously we were appending the wrapper to all the sub-strings passed
via the vector of strings to glShaderSource but we now grow the vector
instead and insert the prelude and wrapper strings into the beginning
and end of the vector respectively so we should only have one copy for a
single shader.

Reviewed-by: Neil Roberts <822ec91a7ef82248fd36162ca8ac3283eba70848@linux.intel.com>

djdeath/cogl,djdeath/cogl-android,djdeath/cogl,gcampax/cogl,gcampax/cogl,djdeath/cogl-android,djdeath/cogl,djdeath/cogl-android,gcampax/cogl,gcampax/cogl