Offer calibration APIs for users with various DPI setups

[chapulina]

We've been having a hard time to support screens with different resolutions and DPIs. Pull request #221 has been open for a long time, but we're failing to find a single solution that applies to all use cases.

My alternative proposal is to provide users with a calibration utility so that they can manually set the scale. See for example this touchscreen calibration tool:

Desired behavior

We could offer a small executable that opens a screen with a few points for the user to click on. Then internally calculate the scaling factor and save it to a file in ~/.ignition/rendering. Every time ign-rendering is loaded, it reads from that file to set the scale. If the file doesn't exist, it uses 1.0 as the scale.

Alternatives considered

We can keep banging our heads against a general solution like we're trying in #221, but there will always be corner cases that we can't test without the hardware.

Implementation suggestion

See above.

Additional context

Related to gazebosim/gz-sim#147