Initial support for Text library.
Text: initial implementation of font and text rendering.

The library uses FreeType for glyph pre-rendering and basic glyph
geometry and HarfBuzz for text layouting (i.e. ligatures, kerning, ...).

Currently all used glyphs must be prerendered into texture atlas, other
glyphs are simply not rendered (although the layouting code handles them
like if they are there).

Text rendering supports UTF-8, although glyph pre-rendering is currently
ASCII only, as I couldn't find any working implementation of Unicode
STL function in recent GCC versions. Will be fixed later.
Renamed *Texture::set*Data() to set*Image().

Better consistency with OpenGL naming.
Text: assert that {ARB,EXT}_texture_rg is supported.

EXT_texture_rg is not available on my OpenGL ES 2.0 implementation, will
find alternative later. At least it now compiles.
Math: matrix/vector rework, part 3: conversion as explicit constructor.

Also updated all dependent classes to follow the change, such as Color
and Rectangle. Backwards compatibility for GCC 4.6 (with lack of support
for delegating constructors) will be done as non-constexpr constructor
using operator=().
Text: prerender font using dedicated function, not in constructor.

The number of possible combinations would explode for no reason when
opening font from different locations will be implemented.
Text: removing duplicate glyphs in Font::prerender().
Text: explicit Font constructor.
Using CORRADE_INTERNAL_ASSERT_OUTPUT() where appropriate.
Doc++
Text: ability to create Font from memory instead of file.
Doc++
Text: mutable TextRenderer implementation.

Allows fast text updates (using buffer mapping). Also fully documented
the workflow.
Text: proper UTF-8 support in Font::prerender().
Text: code cleanup -- merged two loops into one.
Text: ability to compile the library without HarfBuzz support.

The text will be rendered without all the nifty features like kerning
and it will most probably fail on everything non-latin, but HarfBuzz is
currently PITA on some systems.

HarfBuzz usage can be configured using USE_HARFBUZZ CMake option.
Text: using new type aliases in whole Text namespace.
A little bit of const fascism.
Text: minor code cleanup.
Text: prerendering Font for use with distance-field rendering.

Currently there is no builtin shader capable of it, will come later.