Implemented the missing pieces for multiple draw buffers support:
- Fixed Context::getScissoredImage() to use the drawbuffer
  parameter properly
- Enabled setting multiple render targets
- Added dynamic indexing of gl_FragData using a new dynOut flag
  to figure out which channels are being written to.

Change-Id: Id3d95c46a980a698f71e99f7781cc6287e880e9d
Reviewed-on: https://swiftshader-review.googlesource.com/4220Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>