betaflight-configurator Bigger OSD grid support

Consider this a draft/proof of concept + not ready for merge, because:

The firmware doesn't support this feature yet.....
The implementation here should be improved a little - eg: it needs MSP version feature switches; more testing + potentially updating to match however the firmware implements this.

It is configurator side changes to support different grid sizes for the OSD; initially hoping for support along the lines of the HD OSD found in iNav (50 * 18) which is supported by HDZero + others; but the changes make the configurator able relatively easily to support any size.

Not much to it; but in brief I have:

Added a new 'HD' size alongside the existing PAL/NTSC options.
Replaced the single width of 30 (front FONT.constants.SIZES.LINE) with new OSD.constants.VIDEO_COLS, matching the existing OSD.constants.VIDEO_LINES
In CSS, the grid that shows as you drag elements now uses a dashed border instead of a background image, so that we don't need to keep creating new images for every size.
Also a trivial fix for virtual mode; OSD needs to be passed now - guessing the OSD var was previously in the global scope.

It seems to work in so far as I can use the UI, set up a PAL/NTSC OSD as usual + can also set up an HD one including saving/retrieving it from an FC (running BF 4.3) - positions are saved, survive a power cycle etc.

More than happy to own the feature + work on this further should the firmware team decide to implement the feature.