Fix shader compilation on UE4

This patch allows Rive shaders to compile under the conditions of UE4's DirectX 11 config by:

* Disabling some unsupported D3D features when the device feature level is below 11_1
* Changing min16int/min16uint to int/uint, as these types aren't supported
* Adding explicit rounding from float to half values, as this causes warnings which are treated as errors

Authored-by: Elliot Colp <[email protected]>

Diffs=

.rive_renderer

@@ -1 +1 @@
-c25b70544d5fa4a5503a915b0fec80521604e9fb
+0f344fbdba610f9569234e3244d82695d0e86768