Allow copying of ImageWrapper, movement of Image and Trade::ImageData.
Renamed ImageWrapper to ImageReference.

Better reflect what the class is actually about.
Allow conversion of Image and Trade::ImageData to ImageReference.
Document disallowed copy and allowed move better.

Moreover when the deleted function was private, the compilation failed
with (rather cryptic) error that the function is private instead of
noticing that the function is deleted.
Pass image as reference, not pointer to *Texture::image().

Avoids some nullptr-related errors.
Pass image as reference, not pointer to *Framebuffer::read().

Avoids some nullptr-related errors.
Pass image as reference, not pointer to *Texture::set*Image().

Avoids some nullptr-related errors and leads to simpler implementation,
as we need to only handle ImageReference and BufferImage types. All
other Image classes are implicitly convertible to ImageReference and
implicit conversion works only with references, not with pointers. The
usage might be clumsy at this point, as Trade::*Importer classes return
pointer to Trade::ImageData which then needs to be dereferenced.
Hopefully introducing C++14's std::optional (or equivalent) might help
solve that issue.

Also removed "convenience" overloads for setting e.g. 2D subdata of 3D
texture. The convenience overload doesn't cover all subdata cases (only
XY plane, not YZ or XZ ones) and overly complicates the implementation.
This can be done in the future in Image classes themselves.
Reordered *Image parameters.

First format, then type, then size, then data. Seems more intuitive this
way, also the data are laid out in the structure this way.
Math: fixed strict aliasing warning.

Is there _any_ use case of reinterpret_cast where GCC won't loudly
complain?
Initial support for GL tests, initial Buffer test.
Install also bootstrap header for GL tests if BUILD_GL_TESTS is enabled.
Templated version of Buffer::{data,subData}().

More convenient for end user, assert that buffer size is divisible by T
size to avoid some issues.
Trade: Doxygen fixes.
Use emplace() instead of insert() in some places.

Leads to (possibly faster) and shorter code.
Text: interface for font converters.

Provides a way to convert font into different format (either with or
without contents of associated glyph cache) or import/export glyph cache
(i.e. to avoid recreating it from scratch every time).
Text: using utility function instead of own (bugged) implementation.

Also removed the test, as there is now nothing to test.
Text: ability to create glyph cache from all characters.
Text: ability to iterate over GlyphCache.
Text: added AbstractFont::glyph{Id,Advance}() accessors.
Text: forgotten EnumSet operators.