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/DeviceInfo.h

@@ -15,6 +15,7 @@ struct DeviceInfo final {
     String device_name;
     unsigned num_texture_units;
     unsigned num_lights;
+    u8 stencil_bits;
 };
 
 }