This is the first step at trying to un-hardcode the number of drawbuffer
we support. The ultimate goal is to go from currently 4 to 8.
Until now the maximum render buffer supported was the same than the actual
number of render buffer supported. At some point in the future this will
change, it this will have to be dynamic.
This change is to allow rendering a cubemap in a single pass on h/w that
support it. Many mobile GPU support 8 draw buffers.
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