Minimal updates (just the include guards) so Git is hopefully able to
detect the rename and track the history properly.
Everything except Magnum::GL doesn't compile now.
On Windows NVidia drivers the glTransformFeedbackVaryings() does not
make a copy of its char* arguments so it fails at link time when the
original char arrays are not in scope anymore. Enabling *synchronous*
debug output circumvents this bug. Can be triggered by running
TransformFeedbackGLTest with GL_KHR_debug extension disabled.
Also added (currently disabled) ES implementation (provided by
EXT_separate_shader_objects). Unfortunately it's not possible to reduce
the function count, because ARB_separate_shader_objects and
EXT_direct_state_access are completely independent. Also double uniforms
are supported since GL 4.0 and SSO are since 4.1, so we can't omit
old glUniform*() calls for doubles either.