For 565 and 5551 formats, the blend equations were getting
applied on top of colors represented by the top 5 or 6 MSB
only, leading to precision errors.

Fixed this by:
- removing the scaling applied on the source color before
  blending is applied and the related inverse transform in
  the writeColor function
- adding the proper remapping of the color to the full color
  range in the readPixel function

That way, we're always working with full 16 bit range colors
and the blend equations work properly.

Fixes all tests in:
dEQP-GLES3.functional.fragment_ops.interaction.basic_shader.*

Bug: angleproject:4016 b/24332884

Change-Id: I53531b88a8629aaa61929e99efaac38cdb46c834
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/38088Tested-by: Alexis Hétu <sugoi@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>