Depth and stencil buffer blitting

[tobias93]

Adds support for blitting not only the color, but also the depth buffer and stencil buffer. Fixes issue #1404 .

In total, there are three new functions in glium::Surface: blit_buffers_from_frame, blit_buffers_from_simple_framebuffer and blit_buffers_from_multioutput_framebuffer. They behave identical to the already existing blitting functions, there is just an additional argument of type BlitMask that is used to specify which kind of buffers (color, depth, stencil) should be copied.

There are no breaking changes.

There is one last open problem: I wrote a test to check, that a blit with color+depth works. I am not sure how to check the result though, because I could not figure out how to access the values in the DepthTexture2d after the blit. So for now, it only checks that there are no GL errors.

[est31]

OK except for code style nits

[tobias93]

All fixed.

[est31]

Thanks!