When rendering on framebuffer, always do software rendering. This
should make it possible to boot up Plasma Mobile on most devices with
downstream kernels, although terribly slow. Still better than a black
screen though. Tested and working on the samsung-i9100.
We can improve the code and possibly make the rendering mode
configurable per device once we experimented more with:
* llvmpipe vs. softpipe on various devices
* armv7 (around the corner in Alpine)
* a proper display manager like lightdm
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