multimedia: add parameterless QHwVideoBuffer::mapTextures overload

Backends that ignore the pool's old textures implement the
parameterless mapTextures, while the by-reference overload keeps
working as a shim delegating down to it. This makes the unused
parameter explicit at each override instead of threading it
through every backend.

Change-Id: Id71b436656ecf3da6d2920baacacaea79a428803
Reviewed-by: Artem Dyomin <artem.dyomin@qt.io>