xwayland/eglstream: Set ALU to GXCopy for blitting

The EGLstream backend's post damage function uses a shader and
glDrawArrays() to copy the data from the glamor's pixmap texture prior
to do the eglSwapBuffers().

However, glDrawArrays() can be affected by the GL state, and therefore
not reliably produce the expected copy, causing the content of the
buffer to be corrupted.

Make sure to set the ALU to GXCopy prior to call glDrawArrays() to get
the expected result.

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Suggested-by: Michel Dänzer <mdaenzer@redhat.com>
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