Reworked framebuffer attachment enums.

Instead of having pair of functions for depth/stencil and color
attachments this is now solved using specialized class. It is now more
descriptive, as specifying color attachment is now done explicitly using
Framebuffer::ColorAttachment(2) instead of just `2`. Specifying "no
attachment" is now also done explicitly using
Framebuffer::DrawAttachment::None instead of `-1`.

Also updated related documentation.
Added EXT_discard_framebuffer ES2 extension to the list.
Added alias for linking to ES extension functions to Doxyfile.
Implemented ARB_invalidate_subdata GL 4.3 extension.

Framebuffer invalidation is also available in OpenGL ES 3.0 and ES 2.0
using EXT_discard_framebuffer extension.