VideoCommon: fix rounding in TEV.

The hardware backends pass the TEV tests in gxtest with this change.
Merge pull request #277 from magumagu/videocommon-fix-tev

VideoCommon: fix rounding in TEV.
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.
Don't use SDL devices that report invalid ranges.
If Buttons, Axes, Hats, or Balls > 255 then reject it.
Merge pull request #279 from Sonicadvance1/Reject-InvalidControllers

Don't use SDL devices that report invalid ranges.
Add support for the guide button to XInput
Merge pull request #283 from pinumbernumber/xinput-guide

Add support for the guide button to XInput