Refactor shader state setting in the GL backend This is needed in preparation of implementing DispatchCompute, which will need to set some of the same state as draw calls, but not everything. Also move setTextureCubemapSeamlessEnabled to onMakeCurrent, this way it doesn't need to be called on every draw call. Overall this patch doesn't have a measurable performance impact. BUG=angleproject:1442 TEST=angle_perftests, angle_end2end_tests Change-Id: I9958bfe7bf895b3cd805e3146a8f74d28ff2835e Reviewed-on: https://chromium-review.googlesource.com/438349 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>