Improve GUI Configuration and Troubleshooting (#741)

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GUI Configuration and Troubleshooting
=====================================
Video RAM adjustment for big displays
-------------------------------------
Video RAM adjustment for high-resolution displays
-------------------------------------------------
Symptom: I have 4K external display and I when I connect it, I can't click on
anything but an small area in upper-right corner.
**Problem:** You have a 4K external display, and when you connect it, you can't click on anything but a small area in the upper-right corner.
When a qube starts, fixed amount of RAM is allocated for graphics buffer called
Video RAM. This buffer needs to be at least as big as whole desktop, accounting
for all displays that are or will be connected to the machine. So by default it
is as big as needed for current display and an additional Full HD display
(1920×1080 8 bit/channel RGBA). This logic fails when the machine has primary
display in FHD resolution, but after starting the VMs a 4K display is connected.
The buffer is too small and internal desktop resize fails.
When a qube starts, a fixed amount of RAM is allocated to the graphics buffer called video RAM.
This buffer needs to be at least as big as the whole desktop, accounting for all displays that are or will be connected to the machine.
By default, it is as much as needed for the current display and an additional full HD (FHD) display (1920×1080 8 bit/channel RGBA).
This logic fails when the machine has primary display in FHD resolution and, after starting some qubes, a 4K display is connected.
The buffer is too small, and internal desktop resize fails.
Solution: Adjust minimum size of Video RAM buffer.
**Solution:** Increase the minimum size of the video RAM buffer.
```sh
qvm-features dom0 gui-videoram-min $(($WIDTH * $HEIGHT * 4 / 1024))
qvm-features dom0 gui-videoram-overhead 0
```
Where `$WIDTH`×`$HEIGHT` is maximum desktop size the user anticipates. For
example, if user needs 1080p and 4k display side-by-side, that is
`(1920 + 3840) × 2160 × 4 / 1024 = 48600`, or slightly above 48 MiB per-qube.
After adjustment, the VMs need to be restarted.
Where `$WIDTH`×`$HEIGHT` is the maximum desktop size that you anticipate needing.
For example, if you expect to use a 1080p display and a 4k display side-by-side, that is `(1920 + 3840) × 2160 × 4 / 1024 = 48600`, or slightly more than 48 MiB per qube.
After making these adjustments, the qubes need to be restarted.
The amount of memory allocated per-qube is the maximum of:
The amount of memory allocated per qube is the maximum of:
- `gui-videoram-min`
- current display + `gui-videoram-overhead`.
- current display + `gui-videoram-overhead`
Default overhead is about 8 MiB, which is enough for 1080p display (see above).
So the `gui-videoram-overhead` zeroing is not strictly necessary, it only avoids
allocating memory that will not be used.
Default overhead is about 8 MiB, which is enough for a 1080p display (see above).
So, the `gui-videoram-overhead` zeroing is not strictly necessary; it only avoids allocating memory that will not be used.