This CL should not change any visible behaviour. It does the following:
* Extract GLES rendering into separate class GlRectDrawer. This class is also needed for future video encode with OES texture input.
* Clean up current ScalingType -> display size calculation and introduce new SCALE_ASPECT_BALANCED (b/21735609) and remove unused SCALE_FILL.
* Replace current mirror/rotation index juggling with android.opengl.Matrix operations instead.
Review URL: https://codereview.webrtc.org/1191243005
Cr-Commit-Position: refs/heads/master@{#9496}
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