v0.5.0 - Picasso96 / RTG

Picasso96 / RTG support.

• RTG screen modes, 8bpp, from 400x300 upward. They appear below the AGA
  modes in Game Options. Works with both Picasso96 and CyberGraphX.
• On RTG there is no chunky-to-planar conversion at all. OpenTTD already
  renders into a chunky 8bpp buffer, so on an 8bpp RTG screen it goes to the
  display essentially unchanged — and c2p is the most expensive thing the driver
  does on AGA (117 ms for a full frame on an 040/25).
• Machines without a card are unaffected: detection fails safe and falls back to
  AGA, logging why.
• 640x512 AGA removed — 640x480 already covered it.

Requirements — corrected

24 MB Fast RAM, not the 16 MB previously stated. This is measured, not
estimated: the game will not start on 4 MB Fast + 16 MB Z3, but does on 8 + 16,
leaving about 10 MB free. The executable occupies 4.6 MB once loaded and the
sprite cache another 4 MB.

Stripping the binary would not help, which is worth saying because it looks like
it should: only 0.4 MB of the 5 MB file is symbols and debug information, and
AmigaDOS never makes that resident. The weight is code.

Otherwise unchanged: 68030 with an FPU minimum, 68040/40 recommended,
68060/50 for full speed. AGA or an RTG card, AmigaOS 3.0/3.1.

Sound effects are on by default (-s amiga); -s null for silence. Music is
still not implemented.

Building

The RTG path needs the CyberGraphX developer headers, which are not in this
repository — they are copyright phase5 digital products and we have no
redistribution licence for them. patches/README.md says where to get them and
which two need fixing before GCC 6.5 will accept them. Without them the AGA path
still builds; only RTG is lost.

Installing

Unpack anywhere and run openttd from a Shell, keeping splash.dat beside it.
OpenGFX and OpenSFX are bundled — nothing from the original TTD is needed. See
LICENCES.txt for what is in the archive and under what terms.