Fixed default color values for R and RG types

Fixed default color values for R and RG types

By default, in D3D, R, G or B channels default to 1 when no
value is assigned to them. In OpenGL, these channels default
to 0. Added an entry to Conventions to fix this issue.

In dEQP, this fixes all R and RG types tests from:
functional.texture.format.*

Change-Id: Ib5552aa36eaf4e3e1132f016f002250b40436227
Reviewed-on: https://swiftshader-review.googlesource.com/10828
Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>