API: OpenGL - Stencil testing

[bryphe]

A feature that we'll potentially need for efficient rendering / culling in revery (like, for the overflow: hidden case) is a stencil buffer. There's a great overview on Stencil Testing

There's a few items we'd need:

• Add GL_STENCIL_TEST to our variant of options to enable / disable
• Add the ability to clear the GL_STENCIL_BUFFER_BIT
• Add the glStencilMask API
• Add the glStencilFunc API
• Add the glStencilOp API

A fun way to test this would be to render a plane underneath the cube, render the cube's reflection, and use the stencil buffer to constrain the reflection to the plane's dimensions. We could find simpler ways to test this out, too, though.