TextureTools: use `layout(binding = ...)` only if GLSL supports it.
Text: call proper Buffer unmapping function.

The buffer was mapped with mapSub(), we need to unmap it with
unmapSub().
Text: add one more GlyphCache constructor combination.
Shaders: `range` is reserved keyword in GLSL.

Neither desktop GL nor GLES on my NVidia complained. Just Chrome didn't
like it.
Text: fix and improve format checking of passed image in DFGlyphCache.
Shaders: make GLSL code for fullscreen triangle reusable.

Now you can just add the file and call `fullScreenTriangle()`.
Warn about improper Buffer target hint in NaCl.

Slightly eases up the porting.
Platform: minor CMakeLists.txt cleanup.
Added TODO.
Doxygen fixes.
Platform: improve magnum-info in NaCl, add reusable JS and CSS files.

The page now displays download progress, last error and also hints to
the user that JS console might contain something useful. NaClApplication
and WindowlessNaClApplication documentation has been updated with brief
"bootstrap guide".