sim_display D3D11 anaglyph forces alpha=1: browser (weave_frame_first) shows only 3D tiles on black

[dfattal]

Symptom

With the sim-display plug-in (anaglyph, the default mode), the DisplayXR Browser shows only its 3D elements; the rest of the page is black. Seen on the 8K box (DEL4147, RTX 3080), runtime feat/lift-ext v2.21.5-30-gcb5a8607b, DisplayXR Browser build #6 (browser-pvt run 36201668011, Chromium 154.0.8037.17), tools/lift-test-page/native.html with three dxr-lift="auto" elements.

Evidence (dxr_weave_dump_trigger dumps from comp_d3d11_service_weave_submit)

#73 diag: submit n=3 input=3759x4098 ... first=1 (a weave_frame_first present-owner: tiles are drawn back whole-window, transparent gaps are supposed to show the page through).

dump	gap pixel	tile pixel
dxr73_weave_input (3759x4098)	(0,0,0,0)	(44,44,44,255)
dxr73_weave_sbs (7518x4098)	(0,0,0,0)	(44,44,44,255)
dxr73_weave_output (3759x4098, fmt 28)	(0,0,0,255)	(44,44,44,255)

The gaps arrive transparent and leave the DP opaque black.

Cause (likely)

src/xrt/drivers/sim_display/sim_display_processor_d3d11.cpp: the anaglyph pixel shader returns
out_finish(float4(left.r, right.g, right.b, 1.0), uv), which hard-codes alpha = 1.0. Yet the DP advertises itself as alpha-native (sim_dp_d3d11_is_alpha_native, "passes atlas alpha through its blend/anaglyph/SBS pixel shaders to the back buffer — alpha-native by construction"). SBS / squeezed-SBS / quad sample the atlas and keep its alpha; the blend shader should be audited too.

Leia's DP handles this path correctly (the browser works on the Leia plug-in), so this is sim-only. It matters because sim is the no-hardware path for browser/inline-3D and XR_DXR_lift testing (SIM_DISPLAY_FAKE_LIFT=1).

Fix sketch

Anaglyph: carry alpha, e.g. max(left.a, right.a) (or left.a), and check ps_blend_source the same way. Then verify with the dxr_weave_dump_trigger output dump: gaps (0,0,0,0).

Reported by David on the panel; evidence captured by the win-box-8k session (tmp-lift-panel).

[dfattal]

Fix pushed on feat/lift-ext: the D3D11 anaglyph shader now returns max(left.a, right.a) instead of 1.0, and the fake-lift shader keeps the source alpha, so transparent gaps in a present-owner weave stay transparent. Will land on main with the lift branch (or as a cherry-pick if that lands first).