Use GL_RGB as implementation color read format for GL_RGB565
Use GL_RGB as implementation color read format for GL_RGB565

dEQP is very unhappy (and asserts if built in debug) with this since it
has the wrong channel count.

Test: dEQP-GLES3.functional.read_pixels.alignment.*
Change-Id: I35a46ba166d3964091d5e3b3066e3a593952c22a
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/36810
Tested-by: Chris Forbes <c3279c50043e18e9442c1ae071cafa0e7a4ca967@google.com>
Presubmit-Ready: Chris Forbes <c3279c50043e18e9442c1ae071cafa0e7a4ca967@google.com>
Kokoro-Presubmit: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Nicolas Capens <51edc787c30f24c4904e8ffbb5523c3a9f7a451d@google.com>

bkaradzic/SwiftShader,bkaradzic/SwiftShader,bkaradzic/SwiftShader,google/swiftshader,bkaradzic/SwiftShader,google/swiftshader,bkaradzic/SwiftShader,google/swiftshader