Added code to properly render to alphaless floating point formats.

Change-Id: I2e226fb787d8456f61a2946fb9c0d8a9dbb58243
Reviewed-on: https://swiftshader-review.googlesource.com/5128Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>