Checking whether given type can be used for mesh indices or image data
was always done like this, because it was also needed to get OpenGL type
ID for the type:
TypeTraits<typename TypeTraits<T>::IndexType>::glType()
This was cumbersome, now the check is done using function, which
returns the OpenGL type ID directly:
TypeTraits<T>::indexType()
Also replaced TextureType with imageType() and renamed glType() to just
type().
These classes are meant to be used in the same texture updating
functions as Trade::ImageData due to static polymorphism. In addition to
Trade::ImageData, which is read-only, these classes support updating the
dimensions and data. Image2D and ImageBuffer2D can update the data also
from framebuffer.