Make sure readpixels work with 16bit fixed point surfaces

Make sure readpixels work with 16bit fixed point surfaces

Added FBO tests that use unsigned fixed point textures
as render target. Pixels are read from the FBO using
pixel type GL_UNSIGNED_SHORT since that was reported
to have problems on some implementations.

New tests:

dEQP-GLES31.functional.fbo.color.tex2d.r16
dEQP-GLES31.functional.fbo.color.tex2d.rg16
dEQP-GLES31.functional.fbo.color.tex2d.rgba16

Components: OpenGL ES

VK-GL-CTS issue: 2124

Change-Id: Icad477ae62ff6961f5989b097aa3ce73002b8dd8