D3D11 backend: fix rounding in texture encoder.

We need to explicitly round when converting colors from float to uint
because multiplying a normalized float by 255 might not result in a whole
number.  (The exact result here may vary depending on your
drivers/hardware.)

Ideally, we shouldn't be using floating point here, but fixing that is a
much more complicated patch.

Fixes gxtest TEV tests using Intel HD 4000.
Merge pull request #280 from magumagu/d3d-texture-encoder-rounding

D3D11 backend: fix rounding in texture encoder.