LibGL+LibSoftGPU: Implement the stencil buffer

This implements an 8-bit front stencil buffer. Stencil operations are
SIMD optimized. LibGL changes include:

* New `glStencilMask` and `glStencilMaskSeparate` functions
* New context parameter `GL_STENCIL_CLEAR_VALUE`

Userland/Libraries/LibSoftGPU/CMakeLists.txt

@@ -4,6 +4,7 @@ set(SOURCES
     Device.cpp
     Image.cpp
     Sampler.cpp
+    StencilBuffer.cpp
 )
 
 add_compile_options(-Wno-psabi)