Masks set which pixels in the control image should be tested and
an optional tolerance for each pixel. This is done via the colors
in the mask image - white pixels are ignored, black must be an
exact match, and gray levels represent the maximum color component
deviation for that pixel.
This should replace the fragile anomaly images, in that a single
control image with a suitable mask will not be susceptible to
antialiasing differences, etc.
A new script (scripts/generate_test_mask_image.py) is included which
either creates a new mask or modifies an existing mask to handle
an acceptable rendered image.
Ultimately, masking along with multi render checks for specific
platform differences should be flexible enough to meet our needs.
* Recreates composition for every test and therefore removes internal
dependency
* Reorders method calls to make the test pass
* Disable the test until https://github.com/qgis/QGIS/pull/1688
