Render Tests: Separate difference of Color and Alpha

This change fixes confusion situation when the render output
is an RGBA image: the difference in color was not visible in
the report because alpha channel was all zeros. This is due
to idiff performing per-channel difference.

The solution to this problem is to have separate images for
color and alpha difference, which makes it clear where the
difference actually is coming from.

tests/python/workbench_render_tests.py

@@ -56,7 +56,7 @@ def create_argparse():
     parser.add_argument("-blender", nargs="+")
     parser.add_argument("-testdir", nargs=1)
     parser.add_argument("-outdir", nargs=1)
-    parser.add_argument("-idiff", nargs=1)
+    parser.add_argument("-oiiotool", nargs=1)
     parser.add_argument('--batch', default=False, action='store_true')
     return parser
 
@@ -67,11 +67,11 @@ def main():
 
     blender = args.blender[0]
     test_dir = args.testdir[0]
-    idiff = args.idiff[0]
+    oiiotool = args.oiiotool[0]
     output_dir = args.outdir[0]
 
     from modules import render_report
-    report = render_report.Report("Workbench", output_dir, idiff)
+    report = render_report.Report("Workbench", output_dir, oiiotool)
     report.set_pixelated(True)
     report.set_reference_dir("workbench_renders")
     report.set_compare_engine('eevee')