[unit tests] Add multirenderchecker

The multirenderchecker allows to have several images, each with its own set of
anomalies distributed in several subdirectories.
With the help of multiple reference images, it is possible to apply a color
tolerance to each of these
Add color tolerance to pyblendmode test
Add QgsDartMeasurement
[unit tests] Fix blend modes tests on ubuntu precise

Add control images for ubuntu precise
Send to cdash
Fix inverted polygon renderer test
Fix shapeburst test

Move to multirenderchecker
Fix blendmodes test part 2
Fix shapeburst test
Fix shapeburst for travis and docker
Add slight color tolerance to blendmodes test

The renderer seems to produce slight inpredictible variances
Python blend mode test
Formatting
Compositionchecker: Default color tolerance of 1
Don't build grass and staged plugins with travis (saves some cpu cycles)
Fix crash on exit when provider was not cleaned up properly
Fix composerhtml test
Add CMake option to force Qt includes to be treated as system includes
Fix PalLabelingComposer test