Platform: fix console output in Emscripten.

I don't know what I'm doing, but the previous code caused the output
look like "properly" decoded UTF-16.
Use Corrade's Containers::ArrayReference for Buffer::set*Data().

Makes it more usable and allows to remove one overload. The old way is
preserved, but marked as deprecated and will be removed in future
releases.
Text: ported TextRenderer to not require buffer mapping in Emscripten.

The mapping is faked with client-side array, which is then uploaded on
"unmapping".