Viewport position and size is managed separately for each framebuffer
and glViewport() is called in bind() (and also from setViewport(), if
the framebuffer is currently bound) if the viewport differs from
current state. If used only one framebuffer size through whole
application lifetime, glViewport() doesn't need to be called at all.
Optimalizations in Corrade::TestSuite and Corrade::Utility::Debug leaded
to significant reduction of compilation time - on my machine it was
~5:38 before with building of unit tests enabled, now only ~5:00.
Some target platforms supply their own OpenGL headers, thus we cannot
use our own from ES 3.0 and compilation fails.
On the other hand, this will be better for users as usage of unsupported
features will be catched right during compilation and not at runtime.