- the IGNORE_VIEWPORT flag is gone. It wasn't actually needed and what
it was doing was ambiguous.
- removed the backend viewport() API which wasn't actually needed.
- beginRenderPass() now ALWAYS sets the viewport and scissor but
doesn't necessarily clears honoring the scissor (based on the
IGNORE_SCISSOR flag).
- the GLES backend now just sets the scissor when it needs it, as opposed
to always trying to keep it set. we now rely on state tracking to
de-dup the gl calls.
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