Text: returning std::unique_ptr instead of raw pointer.

Avoids memory leaks (I made one in the test, fail).
Add static_assert for deletion of only complete types in ResourceManager.

Borrowed from libstdc++'s std::unique_ptr implementation, on GCC it
generates somehow different error than expected (complains about invalid
application of sizeof() where it should (or not?) print the
static_assert message.
Shaders: ability to specify background color in Vector shader.

Makes it possible to use the shader without blending.
MagnumFont: removed some probably forgotten debug output.
Mesh: warn about improper buffer binding also on Emscripten.

It seems that rebinding is forbidden by law also in WebGL. Also updated
Best Practices docs to note that.