tests: use the clone feature instead of using multiple file loaders
apply-canvas: Start with a copy of value-invert.c
apply-canvas: Register apply-canvas in Makefile.
apply-canvas: Change name and filename of the filter to apply-canvas.
apply-canvas: Update categories and description.
apply-canvas: Remove the OpenCL implementation.
apply-canvas: Ported apply-canvas from GIMP.
apply-canvas: Use the correct format R'G'B'A u8

GIMP is using "R'G'B'A u8" as its format. Since this is a straight forward
port of the existing GIMP code, we have to use the same format for correct
operation.
apply-canvas: Use a variable for the color format in prepare().
apply-canvas: Add support for greyscale + don't assume alpha channel.
apply-canvas: Reorganise wanted format detection according to Daniel_S.
apply-canvas: Take direction and depth as a parameter.
apply-canvas: Do not repeat direction default case values.