Hide Buffer::bind() and Buffer::unbind() from public API.

There should be no need to use these directly (and in some cases it
might be harmful). The bind()/unbind() names will be used for
glBindBufferBase()/glBindBufferRange() later.
Mention usage of flextGL.
Fix compilation on ES.
Doc++
Updated GL headers for OpenGL 4.5.
Initial support for OpenGL 4.5.
Removed APPLE_flush_buffer_range from extension list.

Not mentioned anywhere, doesn't add anything to ARB_map_buffer_range.
Replaced APPLE_vertex_array_object with ARB_vertex_array_object.

We use the core functions without APPLE fallback.
Text: unified and updated pre-rendered glyph set.
Text: updated and fixed Renderer docs.

Thanks again, @wivlaro.
"Fix" flaky TimeQueryGLTest.

Some operations are too fast so the time reported might as well be 0.
Updated related projects in README.
More README updates.
doc: mention usage of CMAKE_INSTALL_RPATH and CMAKE_PREFIX_PATH.
Platform: doc++.
Minor cleanup.
Obvious check is obvious.

Thanks, compiler.
Split Query.h into per-class header.

The old Query.h header is still working, but marked as deprecated and
will be removed in some future release.
Doc++
Support for ARB_conditional_render_inverted (GL 4.5).

That was easy.