Hack for minor precision issues with current implementation. Fixes
8 quality warnings from deqp-gles3 when run against
SwiftShaderGL/Subzero.

Bug: b/151461290
Change-Id: I4a3148add2c0987dcccca96e7c877ea4fb16ca89
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/43470
Kokoro-Result: kokoro <noreply+kokoro@google.com>
Tested-by: Antonio Maiorano <amaiorano@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>