https://gitlab.synchro.net/main/sbbs/-/commit/4668dc2f1f2cd8a9f0b27a84
Modified Files:
src/doors/termgfx/termgfx_termio.c termgfx_termio.h
Log Message:
termgfx: add a truecolor (XRGB) present entry point to termgfx_termio

Add termgfx_termio_present_rgbx(xrgb, w, h), a sibling of the existing
indexed termgfx_termio_present() that takes a native-resolution 32-bit
frame (memory byte order R,G,B,X; the 4th byte is ignored padding;
stride w*4) instead of an 8-bit-indexed frame plus palette. It is the
entry point the upcoming syncrpg door needs, where the source surface is truecolor with no fixed palette of its own.

The new path mirrors present()'s tier / geometry / pacing flow but
produces the encoder input from XRGB:

- sixel tier: nearest-neighbor scale the XRGB frame to packed RGB888 at
the fit ew x eh, quantize it with the shared termgfx_quant_rgb(), then
sixel_encode() the resulting indices + palette -- the same encoder the
indexed path uses.
- JXL tier: scale to RGB888 and jxl_encode() it directly, skipping the
indexed round-trip sst_emit_jxl() does.
- no-graphics tier: nothing to draw, same as present().

Supporting changes, all additive:

- sst_scale_rgbx_to_rgb(): NN XRGB -> packed RGB888 scale helper, taking
the source w/h as parameters (a truecolor source has no fixed
SST_FB_W x SST_FB_H surface).
- g_rgb_buf/g_rgb_cap: hoisted out of the WITH_JXL block so the sixel
truecolor tier can reuse the shared RGB scratch in a no-libjxl build.
- sst_image_rect_src(): the source-parameterized core of sst_image_rect,
which becomes a thin SST_FB_W x SST_FB_H wrapper over it. Passing those
two constants reproduces the wrapper's former behavior exactly (the
source dims feed only the termgfx_geom_fit() call), so the indexed
present()/mouse geometry is byte-identical.
- sst_emit_jxl_rgb(): the tail of sst_emit_jxl() fed already-packed RGB.

The indexed present() path is left byte-identical -- its dedupe /
dirty-rect / retain machinery keys off the indexed g_last_fb surface and
is deliberately not shared; the pacing / backpressure gates, which key
off the tier-agnostic link state, are. A gated truecolor frame is dropped rather than retained (there is no rgbx tick()-retry buffer), since a
truecolor source drives its own redraw cadence.

Compile/link smoke check only: syncscumm links the new symbol into
libtermgfx but does not call it yet (it stays on the indexed path). The functional unit test lands in the following task.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

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