old OpenGL -> fallback to software renderer

[Dirk Hohndel]

> On Oct 29, 2017, at 9:40 AM, Lubomir I. Ivanov <neolit123 at gmail.com> wrote:
> 
> i've made a PR for this at:
> https://github.com/Subsurface-divelog/subsurface/pull/745
> 
> --------------------------------------------------------
> Pull request long description:
> 
> we have a bit of a problem with the map widget...it's QtQuick based
> and QtQuick2 needs OpenGL 2.1 on Desktop. it seems that some users are
> using prehistoric hardware or simply do not have good GPU drivers
> installed. without drivers installed Windows only has OpenGL 1.1.
> 
> But this is a potential problem on all desktop platforms and we need
> to fall back to a software renderer.
> To my understanding Marble did the same.
> 
> Changes made:
> 
> notify the users in README that the desktop version needs OpenGL 2.1 at least.
> attempt to automatically detect older OpenGL and fallback to a
> software renderer.
> --------------------------------------------------------
> 
> please test.

Since I worry about people's ability to test this if it isn't in master (and it
seems self defeating to make it harder to test) I merged this. It's now in
master, new test binaries are in production and should hit the usual
places in the near future.

/D