Add `Stencil::Increment`

[bvssvni]

Stencil::Clip does not always work, such as hierarchical UI. Applying nested clipping pollutes the stencil buffer.

Stencil::Increment is used with nested clipping. Can use Stencil::Inside(depth) to draw inside at depth.

Stencil::Increment should correspond to the following OpenGL commands:

gl::StencilFunc(gl::NEVER, 0 as gl::types::GLint, 255);
gl::StencilMask(255);
gl::StencilOp(gl::INCR, gl::KEEP, gl::KEEP);

[bvssvni]

It works! Closing.