None of this code directly related to implementing CoglTextures, and the
code was needlessly duplicated between the GL and GLES backends. This moves
the cogl_rectangle* and cogl_polygon* code into common/cogl-primitives.c
makes which makes lot of sense since the two copies keep needlessly
diverging introducing or fixing bugs in one but not the other. For instance
I came accross one such bug regarding the enabling of texture units when
unifying the code.