st/mesa: ensure that all pixel paths operation on linear RGB data, not sRGB

st/mesa: ensure that all pixel paths operation on linear RGB data, not sRGB

Before, we were converting between linear/sRGB in glReadPixels,
glDrawPixels, glAccum, etc if the renderbuffer was an sRGB texture.
Those all need to operate on pixel values as-is without conversion.

Also, when setting up render-to-texture, if the texture is sRGB the
pipe_surface view must be linear RGB.  This will change when we
support GL_ARB_framebuffer_sRGB.

This fixes http://bugs.freedesktop.org/show_bug.cgi?id=33353