If we're redrawing the entire viewport, we technically don't need to clear, but on mobile tiling GPUs it's more efficient to let the GPU know we don't need the old content anymore. Currently we're using glClear for that which would be slightly inefficient on desktop, using glDiscardFramebufferEXT would be preferable.

However, on older devices such as Nexus 5X glDiscardFramebufferEXT causes GL errors. We'd need to make its use conditional on capabilities.

Proposed replacement for gl_->Clear:
  GLenum discards[] = {GL_COLOR_EXT};
  gl_->DiscardFramebufferEXT(GL_FRAMEBUFFER, arraysize(discards), discards);