kwin: bugfix nouveau GLES + loose texture bindings in kwin_gles

There seems to be a problem with nouveau GLES if you want to create an
EGLImageKHR more than once in a frame for the same pixmap. This patch
circumvents the problem in the way that it implements tfp the same way
as the mesa example in
mesa/demos/src/egl/opengles1/texture_from_pixmap.c does it. A nice
side effect of this is that it also avoids the overhead of recreating
the texture for every damaged window.

REVIEW: 103303