Add support for using the firmware splash as background for the two-step splash plugin using the ACPI BGRT extension

This pull-req which also contains quite a few commits doing preparation work, adds support to the two-step splash plugin to use the firmware splash as background, this allows a smooth transition from the firmware-splash (typically the vendor logo) to plymouth, see e.g. : https://fedorapeople.org/~jwrdegoede/flickerfree-videos/workstation-bgrt-logo.webm (note this video is based on the old spinner theme).

Besides the BGRT support this commit also addresses a performance issue which I noticed when testing two-step with the spinner theme with the standard background. The fix is included here because it depends on some of the preparation work done for the BGRT support.

Besides adding BGRT support to the two-step splash plugin, this commit also introduces a new bgrt theme which uses this.