Even in the GLES backend, we support RENDERTARGETS=8. This check only
ever considered the first 4.

Bug: b/126719156
Change-Id: I0084aa005f342f06373c53c007bee33268466b92
Reviewed-on: https://swiftshader-review.googlesource.com/c/25808Tested-by: Chris Forbes <chrisforbes@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>