gles: keep blending enabled for shadow buffer blit

[cmeissl]

when rendering in multiple passes we potentially need to blend the shadow buffer with the current content, so lets keep blending active until we are finished.

fixes #1378

[Drakulix]

Doesn't this mess with damage tracking for transparent surfaces? The idea of blitting the shadow buffer into the real framebuffer was to override all parts we touched (which is what we use the stencil buffer for).

Obviously this is only a problem, when we rely on previous contents of the buffer and both can be transparent, so overlay/cursor planes. I feel like this might blend with the previous framebuffer contents, instead of replacing them, when necessary.