The vertex color clamp control is a property of an API,
a lot like gl_rasterization_rules.

The state should be set according to the API being implemented, for example:
   OpenGL Compatibility: enabled by default
   OpenGL Core: disabled by default
   D3D11: always disabled

This patch also changes the way ARB_color_buffer_float is advertised.
If no SNORM or FLOAT render target is supported, fragment color clamping
is not required.