Compositionchecker: Default color tolerance of 1
[unit tests] Fix blend modes test

 * Switched to QgsMultiRenderChecker and added control images for precise
 * Add a minimal color tolerance for slight render inconsistencies
Fix composerhtml test

... that fails on every platform
Fix inverted polygon renderer test
Fix PalLabeling tests

Switch to QgsMultiRenderChecker and add control images for Ubuntu precise where
required.
[unit tests] Make qgis_rendererstest pass

Replace control image