Test: simplify Query tests.

Don't use external resources for *two line* shaders, don't setup whole
framebuffer pipeline just to use transform feedback.
Math: hide macro from Doxygen.

For some weird reason it caused Doxygen to use that as target for
documentation block that is *after* the macro.
Doxygen: use \relatesalso instead of \relates in aliases.

Using \relates causes the documented member to appear as class member
(which is wrong), makes it impossible to link to it without specifying
it as class member (also wrong) and causes Doxygen to just not display
argument list in reference list (why?!).
Hide forward declarations from Doxygen.

Forward declarations of templated types don't have named template
parameters and thus Doxygen (sometimes) used these for documentation. It
then looked like this:

    Magnum::Math::RectangularMatrix<std::size_t, std::size_t, class>

which isn't helpful at all. After the change it looks like this (much
better):

    Magnum::Math::RectangularMatrix<cols, rows, T>
Doc++

Fixed Doxygen warnings, improved some workarounds. Everything now uses
explicit references (except where it doesn't work).
Initial transform feedback implementation.

Full support for EXT_transform_feedback, transform feedback objects
from ARB_transform_feedback2 and equivalent OpenGL ES 3.0 functionality.

Example usage is in src/Magnum/Test/TransformFeedbackGLTest.cpp, I'll
add some example later.
Platform: show transform feedback-related limits in magnum-info.
Test: test transform feedback query in PrimitiveQueryGLTest.