main: don't always clamp pixels read from snorm buffers

From the OpenGL 4.6 spec, section 18.2.8:

"For a signed normalized fixed-point color buffer, each component is clamped to [0,1] if read color clamping is enabled, or if type represents un- signed integer components; otherwise type represents signed integer components, and each component is clamped to [−1,1]."

cc @gerddie