Added missing ARB_shading_language_packing extension.

It contains a few (un)packing functions present in ARB_gpu_shader5, but
isn't mentioned anywhere except GLSL 4.20 changelog (and there it has
its old name, ARB_shading_language_pack2f). It might be supported on
machines where ARB_gpu_shader5 can't be supported (GL3 HW).
Platform: ability to show extensions also for fully supported versions.
Initial glLoadGen work.

Added README, extension list, patch required for it to work on GL 2.1
and the generated files.
Use glLoadGen instead of GLEW.

Removed all known GLEW workarounds, added one small workaround for
missing ARB_texture_compression_bptc. I didn't want to patch glLoadGen
for just four enum values, this way it's possible to use stock one
without any patching (except for missing OpenGL 2.1 support, as stated
in external/OpenGL/GL/README.md).

As one file now replaces both `glew.h` and `glcorearb.h` and it has the
same size as `glcorearb.h` alone , it saves approximately 18k LOC,
resulting in 15 second shorter compilation time (5:03 before, 4:48 now).
Not bad.
Remove `glew` dependency also from all PKGBUILDs.
Fixed compilation on OpenGL ES.

Forgot that glLoadGen is useless there.
Fixed accidental implicit use of explicit constructor.

Spotted with Clang, GCC doesn't care at all. Fixes #12.
Text: fix linker error with Clang.
Updated FindCorrade.cmake from Corrade repository.
More flexible ability to install CMake modules.

FindMagnum.cmake is not installed by default to avoid the need for root
privileges when installing, enable it with WITH_FIND_MODULE CMake flag.
Always install Test/AbstractOpenGLTester.h.

It's counterintuitive when you have to enable `WITH_GL_TESTS` just to
install the header, moreover the dependent projects have no easy way to
detect the header presence, so they just fail on preprocessor error.
doc: updated Getting Started Guide to reflect changes in magnum-bootstrap.
Forgot to update `sdl2` dependency in ArchLinux PKGBUILd.
Add missing `openal` dependency to ArchLinux PKGBUILDs.
Note to self: save before commiting.
Updated toolchains submodule.
Indent the nested ifdefs for better readability.
Move extension loading into Magnum::Context.

Resolves some linker errors in Windows, removes code bloat from Platform
namespace (previously it was copied in _every_ *Application).

Originally this was in Platform namespace because I thought that it
would be possible to have one version of Magnum library for both desktop
and ES and the actual Application would determine which GL edition will
be used. Fun idea, but it would remove the static checks (e.g.
accidentaly using geometry shaders on ES), which isn't worth it.
Export GLLoadGen symbols so they can be used in shared libraries.

Caused linker problems mainly on Windows, where every symbol would need
to be exported manually, on Linux this was done implicitly (or with
`-fvisibility=default`). I now maintain glLoadGen fork at
https://github.com/mosra/glloadgen.git containing changes needed for
Magnum.

The `ogl_*()` functions are used only internally, various global
variables for extension queries are not used at all and thus they don't
need to be exported. I thus enabled `-fvisibility=hidden`.

Fixes #16.
SceneGraph: explicitly export also AbstractCamera symbols.

Caused linker errors with Clang (GCC was fine).