Evas GL: Fix glClear(0,0,0,0) with DR and COPY

Evas GL: Fix glClear(0,0,0,0) with DR and COPY

When glClear is called in direct rendering move (DR), we usually
have to skip the call altogether because clearing out transparency
would erase the pixels in the evas backbuffer. This means Evas
would not be able to blend an RGBA GLView on top of other objects.

But COPY mode should allow Evas GL to poke holes in a window
backbuffer.

Thanks @spacegrapher for the review :)

NOTE: Elm GLView also needs to pass the render op to its Evas.Image.

@fix