### format_utils: Use a more precise conversion when decreasing bits

When converting to a format that has fewer bits the previous code was just shifting off the bits. This doesn't provide very accurate results. For example when converting from 8 bits to 5 bits it is equivalent to doing this: x * 32 / 256 This works as if it's taking a value from a range where 256 represents 1.0 and scaling it down to a range where 32 represents 1.0. However this is not correct because it is actually 255 and 31 that represent 1.0. We can do better with a formula like this: (x * 31 + 127) / 255 The +127 is to make it round correctly. The new code has a special case to use uint64_t when the result of the multiplication would overflow an unsigned int. This function is inline and only ever called with constant values so hopefully the if statements will be folded. The main incentive to do this is to make the CPU conversion path pick the same values as the hardware would if it did the conversion. This fixes failures with the ‘texsubimage pbo’ test when using the patches from here: http://lists.freedesktop.org/archives/mesa-dev/2015-January/074312.html v2: Use 64-bit arithmetic when src_bits+dst_bits > 32 Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>

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