This omission was noticed by Benjamin Otte. Add a premultiply
uniform to the external shader, and add a separate premultiply
shader for the non-external case.
This shader uses samplerExternalOES to sample an external texture
and blit it into a 'normal' texture. It only works in GLES, but
we won't use it outside of GLES.
This allows the gskglprograms.defs a bit more control over how a shader
will get generated and if it needs to combine sources. Currently, none of
the built-in shaders do that, but upcoming shaders which come from external
libraries will need the ability to inject additional sources in-between
layers.
