We chose 8 renderbuffer because it seems to be commonly supported
on mobile gpu and is more than 6 which is needed for generating
cubemaps. 4 was the minimum guaranteed by ES3.0.
The main changes here are:
- TargetBufferFlags is now a uint32_t instead of uint8_t
- The number of render buffer supported by filament and supported by
the hardware can now be different. Applications/users now must
query this value if more than 4 is needed.
- added a backend query (used gl terminology)
This is a redo of a previous attempt. There was an issue in the GL
backend that caused MRT to fail on some devices.
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