libobs: Draw Darken sources over white so transparency keeps the background

[rdoupe]

Fixes #1404

What and why

A source set to Darken blending now leaves the background where it is transparent. Before, it drew black there and cut a hole in the scene's alpha (upstream obsproject/obs-studio#6591, reported by SuslikV; closed upstream as not planned, though the blend-mode author agreed it is a bug).

• Darken uses GS_BLEND_OP_MIN, and Direct3D and OpenGL ignore the blend factors for MIN and MAX: each channel becomes min(source, destination) on premultiplied colour, and a transparent pixel (0, 0, 0, 0) always wins.
• Darken items always render through their item texture (item_texture_enabled(): non-default blending). That texture is now cleared to opaque white instead of transparent black for Darken items, so the source is drawn over white. A transparent pixel becomes white with alpha 1 and the minimum keeps the background's colour and alpha. An opaque pixel is the same as before. A partly transparent pixel is darkened as the source over white would be, which is the workaround users were given upstream and on the forum (re-export the image on white).
• Only Darken items change; every other blend mode clears as before.

Headless (private Xvfb, private D-Bus, fresh HOME), driven over obs-websocket: a light grey (209) colour source, a PNG over it whose left half is transparent and whose right half is opaque grey (128), scene screenshot:

Build	Darken, transparent half	Darken, opaque half	Normal / Lighten / Multiply
v0.1.0-beta.20261014 AppImage	0, 0, 0, alpha 0	128	unchanged
master 3fa2833	0, 0, 0, alpha 0 (test: luma 16)	128	unchanged
This branch	209, 209, 209, alpha 1	128	unchanged (209 / 209 / 104 on the right)

RED → GREEN

• RED: 9db475d test/cmocka: Add failing test for Darken over a transparent source. test_darken_transparent puts an opaque light grey background on the program and over it a box that is transparent in its left half and opaque darker grey in its right half, runs a raw video encoder on the program and compares each half's mean luma with Darken and with Normal (where the box is opaque it is darker than the background, so the two must agree). On master it fails 1 of 2: darken_keeps_background_where_transparent (left half luma 16 with Darken, 180 with Normal). Needs core video: runs under xvfb-run on Linux, skips without a display.
• GREEN: e34bbce libobs: Draw Darken sources over white so transparency keeps the background. 2 of 2, three runs; test_scaled_mix_alpha (Subtract) still passes. Full build (C libobs lane) and full ctest -j4 (private Xvfb, private D-Bus, fresh HOME, system libobs-opengl masked): 304/304; the only skipped case is open_crashes_without_obs_core, as on master. The scene renderer is C in both libobs lanes.

Checklist

• Base is master
• I claimed the issue, or no one else has an open PR for it
• Characterization test (if the unit had none), then RED, then GREEN commits
• (Optional) Local gate passed and the pushed head is stamped; otherwise PR CI runs the full suite
• Warning or flag changes (-W..., ratchets): none
• Intentional differences from C listed in the parity test header and rust/README.md (ports): not a port

AI note: Claude Code (Claude Opus 5.5) wrote the code, the test, the commit messages and this text under the submitter's direction, and ran the tests above on the submitter's machines.