Configurable image resolution

[avdstaaij]

The resolution of the output image should be configurable.

Ideally, it should be possible to specify only the resolution of one image axis (width or height) and have the other one be determined automatically (this would also replace the current --trim flag). This may however be difficult to get right: we have to set the resolution before rendering, but we cannot easily find out the aspect ratio that the trimmed rendered image will have before actually rendering it.