riders: missing brightness control

[jotego]

The brightness control shown on the schematics PDF page 17 is not implemented yet.

Reference of background dimmed vs normal sprites on real PCB here

[jotego]

Implementation in 6a2c5050 shows bright sprites but the background is dimmed.

The output impedance of the 053535 is yet unknown but the brightness RDAC would affect black levels if implemented by translating the output voltages directly. The key here is that the TV set clamps to the dark level during blanking, so black is always black and the brightness RDAC is really limiting the maximum brightness but not the blackness.

[jotego]

Added NODIMMING macro to disable dimming for releases until it works correctly:

`ifdef JTFRAME_RELEASE
    `define NODIMMING
`endif

This is always set for MiST/SiDi to allow for memory saving. It could be replaced by a simple dimming without LUT on those systems.

[jotego]

simulation scene 3 shows how different dim settings make the scene look blocky and wrong. The hay that appears dark comes scroll B, the bright hay is scroll A

Shade fixed in 967f126452964a9210cf1a911507fbce6a961546

[jotego]

scene 10 should look with a dimmed background and bright fix layer. But everything looks dimmed instead.

from PCB: