Update README.md with Ubuntu setup instructions

[orao]

I had trouble following the existing instructions on my Ubuntu 20.04 installation and have updated the README.md with what I found worked for me.

Please review and let me know if there is a better way to set this up.

I'm thinking that it might be worth considering including the source dependencies as submodules.

[crwper]

Sorry it's taken me so long to get to this. Lots of good stuff here. Going through this process on a new Ubuntu install, I ran into a couple of things I wanted to ask you about.

First, rather than manually copying wwWidgets libraries, would it be better to use make install, as you did later on, i.e., instead of these commands (from my original version, not your changes):

sudo cp widgets/libwwwidgets4.so /usr/lib/libwwwidgets4.so
sudo cp widgets/libwwwidgets4.so /usr/lib/libwwwidgets4.so.1

Would something like this be better?

sudo make install sub-widgets
sudo ln -s /usr/local/lib/libwwwidgets4.so /usr/lib/libwwwidgets4.so
sudo ln -s /usr/local/lib/libwwwidgets4.so /usr/lib/libwwwidgets4.so.1

Second, to clone libVLC, you use:

git clone git@github.com:vlc-qt/vlc-qt.git

Is there any advantage compared with the following commands (a bit more familiar for me)?

git clone https://github.com/lib-vlc/vlc-qt.git
cd vlc-qt

[orao]

sudo make install sub-widgets

The make install would be better. I'm not sure if we also need the --install-prefix on the fresh Ubuntu clone but I can try it out.

Regarding the:

git clone git@github.com:vlc-qt/vlc-qt.git

The https version may be better as it doesn't require the ssh key so that's good spotting on your part.

I was also considering potentially writing a ./configure script or something similar to allow a quick setup of the repo on a Linux distro that would attempt to do these steps automatically but haven't had time to try implement it.

I have had to reinstall my OS and lost my Ubuntu install, so I have a chance to try this again fresh.

[orao]

Without mesa-utils we seg. fault somewhere in OpenGL code. After we install mesa-utils and setup the DISPLAY env. var. we get:

user@DESKTOP:~/repos/flysight-viewer-qt/build$ ./FlySightViewer
qt.qpa.xcb: could not connect to display localhost:0
qt.qpa.plugin: Could not load the Qt platform plugin "xcb" in "" even though it was found.
This application failed to start because no Qt platform plugin could be initialized. Reinstalling the application may fix this problem.

Available platform plugins are: eglfs, linuxfb, minimal, minimalegl, offscreen, vnc, xcb.

Aborted

I'm now installing ubuntu-desktop expecting it to fix the remaining issues.

[orao]

I'm redoing these instructions on WSL2 now and I'm hitting the wslgl issue 1214 now which I need to side-step using:

export GALLIUM_DRIVER=llvmpipe

[orao]

Rebased and squashed the commits.

[crwper]

I'm slogging through this again myself on a new Ubuntu install. I'm running into trouble when making VLC-Qt, as CMake isn't able to find the Qt package configuration file. Any chance this sounds familiar?