Also added support for image saving.

This allows background to take precedence over objects.
This completes the CGB ACID 2 test compliance.

Previously only new ROM loading operations would take into consideration the auto-mode.
This fix affects the SDL frontend only.

In the SDL version.

Created infrastructure that allows the usage of the serial device to capture the result of the tests.
Then the test is executed agains a pre-defined number of cycles and the result is compared against expectation.

This makes it possible to select CBG or DMG mode taking into consideration the CGB flag in the ROM file.