The lv_drivers provided monitor driver supports a `MONITOR_ZOOM`-factor
which scales the window by the set factor. This is 'useful when
simulating small screens'.
The zoom can be set as cmake configuration setting `-DMONITOR_ZOOM=1`.
Probably even more usefull for high-dpi screens where 240 pixels is
really tiny.
When pressing `i` create a screenshot of the current InfiniTime screen
by dumping it to a timestamped png or bmp file like
`InfiniSim_2022-03-08_203421.png`.
Add a new configure option `WITH_PNG` with default `ON`. When switched
to `OFF` the screenshots will be created in `bmp` format.
Fixes: https://github.com/InfiniTimeOrg/InfiniSim/issues/5
