Blitter fix

Conversion from a float format to a non float format was
broken in the blitter because the clamp operation, either
to the [0,1] range or the [-1,1] range, was happening
after the scaling operation, so non float types were
becoming either -1, 0 or 1, which was wrong.

Change-Id: I3e1290313043fc49030454916b6e4ea6666f8343
Reviewed-on: https://swiftshader-review.googlesource.com/4290
Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>