Math: use both const and constexpr.

Spotted by Clang.
Text: don't use explicit constructor implicitly.

Spotted by Clang.
SceneGraph: Clang linker fixes.
Specify CMake module dir before trying to find Corrade.

We need file from that module dir to find it.
No need to explicitly specify static build for NaCl's newlib.

It is enabled by default.
Updated FindCorrade.cmake from Corrade repository.
Text: fallback to OES_mapbuffer if EXT_map_buffer_range is not supported.

The renderer emits warning about crappy hardware, similarly to what
GlyphCache does.
Added CHROMIUM_map_sub NaCl-specific ES2 extension to the list.
Implemented CHROMIUM_map_sub for Buffer.
Text: use CHROMIUM_map_sub rather than OES_mapbuffer.

OES_mapbuffer isn't available in (my) NaCl anyway and CHROMIUM_map_sub
should be faster.