Fix support for variable number of render targets.

Bug 19353282

Change-Id: I4e99589477de32bb8004feec673e1dbddb675047
Reviewed-on: https://swiftshader-review.googlesource.com/5144
Reviewed-by: Alexis Hétu <sugoi@google.com>
Tested-by: Nicolas Capens <capn@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>