Fixed compilation of Renderer on OpenGL ES.
Call glClearDepth() if glClearDepthf() is not available.
Initial support for context flags.
Doc++
Why the hell did I write this?
Support for ARB_robustness.

Robust *Framebuffer::read() access, ability to query robust buffer
access behavior in Context::flags(), ability to check graphics reset
status and reset notification policy in Renderer.
Added convenience member alternative for AbstractImage::pixelSize().
Reading parameters from image in *Framebuffer::read().

There will be more parameters like row length etc. which would need to
be passed too. This function now expects properly formatted empty image
which it fills with data.
Don't sort member docs in Doxygen so \overload works as expected.
Refactored AbstractTexture internals.

Naming implementation functions similarly to GL function they are
wrapping, naming internal API functions similarly to public API.
Ability to retrieve texture images.