Raise minimum supported Feature Level to 9.3

[walbourn]

The DirectX Tool Kit has supported all feature levels 9.1 and up, primarily to support Surface RT and Surface RT 2 devices. Since these devices will not be getting upgraded to Windows 10, and given the fact that the vast majority of PCs have 10.0 or better hardware, we will be retiring support for FL 9.1. There are almost no 9.2 devices of relevance today, but Windows phone 8.1 support requires FL 9.3.

[walbourn]

For more information on Direct3D Hardware Feature Levels, see this post.

The primary benefit this would provide be able to build all pixel shaders using ps_2_x rather than being limited to the ps_2_0 model.

Note that DirectX Tool Kit for DirectX 12 can assume a base feature level of 11.0 as there are no DX12 drivers available or planned for older video cards.

[walbourn]

Note that building the shaders with 9_1 vs. 9_3 makes no difference at all in terms of code size, and for the few shaders where a higher feature level would help it's going to have to be 4_0. As such, perhaps it's best to just leave them using 9.1 but know that Feature Level 9.1 and 9.2 aren't really in practical use.

If and when we drop 9.x entirely, it would make the binary blobs a lot smaller, but keeping 9.3 is important for UWP and Windows phone 8.1

[walbourn]

UWP on Xbox One for DirectX 11 requires Feature Level 10.0 even in 'game' mode.