Allow arbitrary scaling

[kamicane]

Basically, a way to manually specify the exact size at which to render the game and the user interface, while also scaling mouse movements.

The main use case would be using ScummVM on a CRT where, due to hardware limitations, one is forced to use super resolutions.

Using Graphics mode = "Normal" and Stretch mode = "Stretch to window" produces the correct visual results when using a 1920x200 (15khz) or 1920x400 (31khz) resolution exactly, however the mouse movement isn't scaled to the X axis, resulting in a very slow horizontal movement. Another useful feature would be automatic maximum integer scaling on both axes, so that using a 1920x240 or 1920x480 resolution would always produce correct vertical scaling.

Tested using ScummVM 2.1.0git (Jul 3 2019 00:44:03) compiled on a raspberry pi 3 with ./configure --disable-all-unstable-engines --enable-vkeybd --enable-release --disable-debug --enable-keymapper --disable-eventrecorder --host=raspberrypi

[sev-]

It is now implemented in OpenGL renderer.