Add a paravirtualized framebuffer for the Quadra 800

[SolraBizna]

This PR adds a NuBus-flavored paravirtualized framebuffer, complete with a declaration ROM and QuickDraw driver. It also adds support for unplugging the DAFB framebuffer.

• -M q800,fb=none unplugs the "monitor" from the DAFB, making the Mac headless.
• -M q800,fb=qemu unplugs the DAFB and adds a nubus-qfb configured by the -g option.

Regardless of the value of fb, additional framebuffers can be added with -device nubus-qfb. Width, height, and depth can be specified per-card, e.g. -device nubus-qfb,width=960,height=600,depth=16. Since the Quadra 800 has three NuBus slots, three paravirtualized monitors can be added. If the DAFB is utilized, a fourth monitor is possible.

Advantages of the paravirtualized framebuffer:

• Different set of glitches
• Emulated CPU wastes less time programming real hardware registers
• Support for Thousands
• Arbitrary resolutions
• Multiple monitors
• Gamma correction

The QuickDraw driver was written to function with any version of the System Software that is compatible with the Quadra 800. It was tested with System 7.0.1, MacOS 8.1, and A/UX 3.1.1. All features of the driver work in all tested configurations, though getting multiple monitors working on A/UX takes a little bit of song and dance. All specified requirements for QuickDraw drivers are met, with one exception. If Slot Manager 2 or 32-bit QuickDraw are missing from the ROM, nubus-qfb can't be used as the startup screen. The Quadra 800 ROM has both, so this will only be a problem if we add older Mac machines later, notably Macintosh II variants.

The source code to the driver and the declaration ROM can be found here, under a liberal license.

[github-actions]

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