swrast: fix readpix clamping

swrast: fix readpix clamping

Broken with e5c6a92a12b5cd7db205d72039f58d302b0be9d5. (ARB_color_buffer_float)

Clamping should occur if type != float, otherwise the MSBs of the resulting
pixels are killed off. For example, reading back LUMINANCE = R+G+B can be
greater than 0xff, but the result is naturally masked by 0xff
for UNSIGNED_BYTE, leading to bogus results.

The following bug report seems to want clamping to occur if type == half_float
too. Not sure what's correct.

Bug: [bisected pineview] oglc case pxconv-read failed
https://bugs.freedesktop.org/show_bug.cgi?id=35852

Tested by: Fang Xun <xunx.fang@intel.com>
Reviewed-and-tested-by: Ian Romanick <ian.d.romanick@intel.com>