Add SM16208 driver and FOUR_SCAN_40_80PX_ZIGZAG8 scan type for 80x40 4-scan panels

[pkilar]

What

Adds library support for a custom 80×40, 1/10-scan HUB75 panel whose driver ICs fold two logical rows into one 160-clock shift register per colour lane and reverse the column order inside every 8-pixel segment. Standard scan types do not drive it correctly.

The panel

An 80×40, 1/10-scan module (PCB silkscreen L920F 10S 80x40 1921-V3.0+) driven by SM16208 constant-current shift registers:

SM16208SJ shift-register IC	Back: 3×6 driver grid, 1/10-scan 4-band layout
L920F 10S 80x40 1921-V3.0+ silkscreen	Live via FOUR_SCAN_40_80PX_ZIGZAG8 + SM16208 — correct geometry, no ghosting

Changes

• FOUR_SCAN_40_80PX_ZIGZAG8 (VirtualMatrixPanel_T) — coordinate remap onto the folded 160×20 DMA surface. New else if constexpr branch; the existing FOUR_SCAN_40_80PX_HFARCAN branch is untouched.
• SM16208 shift driver — this panel's SM16208 constant-current chips latch on the inverted shift clock, so the driver sets clkphase = false, mirroring the existing MBI5124 case.
• doc/Panel_80x40_4Scan_ZigZag8.md (+ two SVGs) documenting the panel layout and pixel ordering; linked from the README scan-types and chips lists.

Usage

HUB75_I2S_CFG mxconfig(160, 20, 1);          // 160×20, NOT 80×40
mxconfig.driver = HUB75_I2S_CFG::SM16208;    // sets clkphase = false

auto *dma = new MatrixPanel_I2S_DMA(mxconfig);
dma->begin();

VirtualMatrixPanel_T<CHAIN_NONE, ScanTypeMapping<FOUR_SCAN_40_80PX_ZIGZAG8>, 1>
    panel(1, 1, 80, 40);
panel.setDisplay(*dma);
panel.drawPixel(x, y, color);                // logical 80×40 coordinates

The DMA must be configured 160×20, not 80×40 — the mapping targets the folded surface; an 80×40 config drops every mapped x ≥ 80 and never lights rows 20–39.

Testing

Verified on a physical 80×40 panel of this type: correct geometry across all rows and columns, and no ghosting — selecting the SM16208 driver (clkphase = false) fully resolved the ghosting seen with the default clkphase = true.