Set both TARGET_NACL and MAGNUM_TARGET_NACL in CMakeLists.

Being consistent with all other options, all of them are both
unprexfixed and prefixed in CMake (and then only prefixed versions are
saved into `magnumConfigure.h` file).
Platform: no need to include GL headers just for GL_TRUE.

It is, always was and always will be defined as 1, so why bother.
Fix comma at the end of enumeration list.

I know C99/C++11 allows this, but for me this is as bad as wrong
whitespaces or superfluous `;` characters.
Platform: fix warning about undeclared function.
Doc++
Fix OES_required_internalformat-related enums in AbstractTexture.

Some values are present in ES2 itself, thus no need to use the ones
defined by the extension for ES1.
Clarified AbstractImage Format and Type enum documentation.

Each value is now documented if it can be used for framebuffer reading
or texture data only.
Updated external OpenGl headers, added missing `GLES2/gl2platform.h`.
Better usage of external OpenGL headers.

Moved them to `OpenGL/` subdirectory, allowing them to be included
explicitly with e.g. <OpenGL/GLES2/gl2ext.h> overriding the system
<GLES2/gl2ext.h> header. Our versions of the headers are thus now
explicitly included in `OpenGL.h`, but they can be also included using
no-prefix path if no system version is available. It might break some ES
platforms, they will be fixed when found.

The headers are now installed into `Magnum/OpenGL` (not into any
artificial `external` directory). Now also installing GLES2 headers for
OpenGL ES 2 (previously ES3 headers were installed for both ES2 and
ES3).
Support for ES2 extension NV_framebuffer_blit.
Support for ES2 extension ANGLE_depth_texture.
Support for ES2 extension NV_texture_border_clamp.
FindMagnum: provide MAGNUM_APPLICATION_{LIBRARIES,INCLUDE_DIRS} aliases.

Convenience aliases to variables related to one particular *Application
component, defined if and only if exactly one *Application component is
requested and found. Meant to be used to simplify porting when each
platform uses different *Application library.
Fixed NaCl issues with gl2.h.

Headers gl2.h and gl2ext.h shipped with NaCl are different to the
official ones, which is causing linker issues, thus using NaCl's own
gl2.h. They are otherwise similar, thus it should cause no compatibility
issues.

On the other hand, gl2ext.h shipped with NaCl is slightly outdated with
some recent extensions missing. We are including the NaCl's one and then
the official one over it (undefining the include guard). The symbols are
guarded also by extensions, so it should cause no conflicts.