Remove unused code from Texture2D.DirectX and do a minor tidy up. Fixes #2306
Tidy up the usings also.
Clamp MediaPlayer volume to [0, 1] range. Fixes #2362
Fix WM_QUIT drain in TestGameBase

According to MSDN, WM_QUIT causes GetMessage to return 0. Checking for
this fixes a hang issue when running the Windows build of MonoGame on
Windows 8.1.
Merge pull request #2388 from thefiddler/win_test_hang

Fix WM_QUIT drain in TestGameBase
Merge pull request #2387 from danzel/fix-2362

Clamp MediaPlayer volume to [0, 1] range
Merge pull request #2386 from danzel/fix-2306

Remove unused code from Texture2D.DirectX and do a minor tidy up
Merge back from develop
This DXT modification was breaking XNB loading. It seems wrong anyway.