mesa: GL_ARB_half_float_pixel is not optional

mesa: GL_ARB_half_float_pixel is not optional

Almost every driver already supported it.  All current and future
Gallium drivers always support it, and most existing classic drivers
support it.

This only changes radeon and nouveau.

This extension only adds data types that can be passed to, for example,
glTexImage2D.  It does not add internal formats.  Since you can already
pass GL_FLOAT to glTexImage2D this shouldn't pose any additional issues
with those drivers.  Note that r200 and i915 already supported this
extension, and they don't support floating-point textures either.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>