gitignore: ignore baseline image files

These should be converted to `.sha512` files by CMake machinery, but
developers that miss running `SetupForDevelopment.sh` may accidentally
end up committing the files directly.

.gitignore

@@ -2,6 +2,9 @@
 .ExternalData_*
 .ExternalData/config
 
+# Do not commit baseline image files.
+**/Testing/Data/Baseline/*.png
+
 # Exclude compiled Python objects.
 *.pyc
 *.pyo