[cogl-gles2-wrapper] Convert texture unit settings to be a static sized array

[cogl-gles2-wrapper] Convert texture unit settings to be a static sized array

Previously the texture unit settings were stored in growable GArrays
and every time a new texture unit was encountered it would expand the
arrays. However the array wasn't copied when stored in a
CoglGles2WrapperSettings struct so all settings had the same
array. This meant that it wouldn't detect that a different program is
needed if a texture unit is disabled or enabled.

The texture unit settings arrays are all now a fixed size and the
enabledness of each unit is stored in a bit mask. Therefore the
settings can just be copied around by assignment as before.

This puts a limit on the number of texture units accessible by Cogl
but I think it is worth it to make the code simpler and more
efficient. The material API already poses a limit on the number of
texture units it can use.