I went through renaming this on many places quite some time ago, but
this one slipped through. Now that UBOs will be a thing, rename to
EXPLICIT_BINDING instead of EXPLICIT_UNIFORM_BINDING.
Everything what was in src/ is now in src/Corrade, everything from
src/Plugins is now in src/MagnumPlugins, everything from external/ is in
src/MagnumExternal. Added new CMakeLists.txt file and updated the other
ones for the moves, no other change was made. If MAGNUM_BUILD_DEPRECATED
is set, everything compiles and installs like previously except for the
plugins, which are now in MagnumPlugins and not in Magnum/Plugins.
Even if the extension specification describes the minimum required
version as GL 2.1 (or doesn't mention it at all), most compilers can't
handle `layout(...)` before input/output declaration. These drivers mark
the extensions as supported on GL < 3.1, but then can't handle them at
all: Mesa fails to compile it with GLSL 1.20, NVidia fails with GLSL
1.30.
Also updated `EXPLICIT_*` defines in Shaders/compatibility.glsl to
reflect this change.