old OpenGL -> fallback to software renderer

[Lubomir I. Ivanov]

i've made a PR for this at:
https://github.com/Subsurface-divelog/subsurface/pull/745

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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.
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please test.

thanks
lubomir
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