Remove totally useless introduction paragraph in building documentation.
We don't need Graphviz for Doxygen docs anymore.

The diagrams were useless anyway.
Mention that both target and native Corrade is needed for crosscompiling.
Improve sectioning in building documentation even more.
Fixed segfault in AbstractTexture::bind().

Caused in 6ee2745503bf056185f111d6faeb1f2491210af6, I accidentaly
removed code that initialized the array to proper length. Again I
experienced how useful unit tests for GL functionality would be.