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.
* Not requiring ARB_texture_storage, as Texture::setStorage() has
already implemented fallback.
* Not requiring EXT_texture_rg for single-channel texture on ES2, as
this extension might not be available everywhere (unlike in ES3
desktop OpenGL, where Mesa 8/9 with OpenGL 2.1 supports it), fallback
to Luminance in GlyphCache and RGB in DistanceFieldGlyphCache,
because Luminance might not be renderable everywhere.
* Re-enabled building of Text library in all ES PKGBUILDs.
Vladimír Vondruš