configurable vrefresh

[totaam]

Some applications may be querying the virtual screen's virtual outputs (#56) to get the vertical refresh rate in order to synchronize with it.

f37e274 makes it possible to override the default which was 50Hz.
We should:

• add a command line switch (to set the default on the server, to override detected value on the client)
• expose the vrefresh per-output in the screen definition
• use the global vrefresh value if that's not provided
• perhaps bump the max clock so we can have high resolutions without bumping into the 230MHz clock limit (and expose this somehow so the code knows what limits to use)

[totaam]

I don't think bumping the max clock was really needed, but in any case:

[ 39929.781] (**) DUMMY(0): Max Clock: 30000000 kHz
[ 39929.781] (II) DUMMY(0): dummy_monitor: Using hsync range of 1.00-30000.00 kHz
[ 39929.781] (II) DUMMY(0): dummy_monitor: Using vrefresh range of 1.00-300.00 Hz
[ 39929.781] (II) DUMMY(0): Clock range:  11.00 to 30000.00 MHz

That should be enough for some time.

[totaam]

The server side is working.
New tasks:

• set_initial_resolution needs doing...
• the shadow server's refresh_delay should honour the new refresh-rate command line option

Minor issue: the modeline is being rounded down somewhere:

3840x2160@30  29.00*

We calculate a modeline for refresh-rate: 29998 and correctly name it @30 (rounding to nearest) but somehow xrandr gets 29Hz instead..

[totaam]

Now with support in the client, here are the resolutions that are generated from my 4K60 monitor (technically 59.996Hz) using auto, 50% and 80% for --refresh-rate:

   3840x2160@60  59.00* 
   3840x2160@30  29.00  
   3840x2160@48  47.00  

And since the code is shared, I get the exact same result when using the --refresh-rate switch on the server instead.
Note: using it on both client and server would have a cumulative effect. ie: 50% applied twice is 25%.

[totaam]

The commit above ensures that the refresh-rate specified is honoured when setting the initial resolution and when adding new ones via the control channel.

Note: refresh-rate does not override resize-display:
ie: xpra start --resize-display=4K60 --refresh-rate=30.
In this case, the initial resolution will be 4K at 60Hz, only resolutions added after that will use 30Hz.