Ability to build only some plugins.

Everything is disabled by default, as there would be many dependencies
otherwise.
Updated toolchains submodule.
Updated FindCorrade.cmake from Corrade repository.
Specify CMake module path before trying to find Magnum.

We need files from that path to find Magnum.
Ability to build the plugins either as dynamic or static.
OpenGL ES build fix.
TgaImporter: return Luminance if EXT_texture_rg is not supported in ES.

Also documented the behavior.
MagnumFont: no need to use DistanceFieldGlyphCache.

We are providing already done font with parameters that GlyphCache can
handle. Moreover in ES DistanceFieldGlyphCache might expect RGB input
images because Luminance is not renderable in most cases, thus it
wouldn't work with Luminance TGA input.
Added FindMagnumPlugins.cmake module.

Usable mainly when linking with static plugins or when externally
developing plugin depending on some of these.
Added NaCl PKGBUILD.