third_party/mesa: track mesa-wink's wink branch, at 9016782

The submodule named the fork's gex-virgl branch (4eeb32a), while the
browser's Mesa (gex-wgpu-replay, the path Windows programs use) has been
built from wink since S12's milestone (c). It now records wink at 9016782:
c6a33e6 plus S12's two gex_wgpu patches (the stream uploader maps
explicitly, not persistently: 59 MB -> 19 KiB of writeBuffer per Noita
frame; subdata into a buffer the batch reads is staged and copied in the
batch: ~60 -> ~12 submits per frame).

wink has no gex-virgl-replay target (that driver is four commits on
gex-virgl), so web/build-mesa-virgl-wasm.sh now says so up front and takes
GEX_MESA_SRC to build from a gex-virgl checkout instead.

Co-Authored-By: Claude Opus 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01PAzyvifGuxHm26BuKQ1ggv

.gitmodules

@@ -7,4 +7,4 @@
 [submodule "third_party/mesa"]
 	path = third_party/mesa
 	url = https://github.com/MercuryWorkshop/mesa-wink.git
-	branch = gex-virgl
+	branch = wink

web/build-mesa-virgl-wasm.sh

@@ -18,7 +18,7 @@
 set -eu
 
 ROOT=$(CDPATH='' cd -- "$(dirname "$0")/.." && pwd)
-MESA=$ROOT/third_party/mesa
+MESA=${GEX_MESA_SRC:-$ROOT/third_party/mesa}
 OUT=${1:-${TMPDIR:-/tmp}/gex-mesa-virgl}
 BUILD=$OUT/build
 PUB=$ROOT/web/public/gex
@@ -31,6 +31,15 @@ command -v emcc >/dev/null 2>&1 || { echo "need Emscripten" >&2; exit 1; }
     echo "initialize third_party/mesa" >&2
     exit 1
 }
+# The submodule tracks mesa-wink's `wink` branch, which has the WebGPU driver
+# but not the virgl replay (gex-virgl-replay): that lives on the fork's
+# `gex-virgl` branch. Say so rather than fail deep in meson.
+grep -q gex-virgl-replay "$MESA/src/gallium/targets/gex-wasm/meson.build" 2>/dev/null || {
+    echo "third_party/mesa has no gex-virgl-replay target: it is on mesa-wink's" >&2
+    echo "gex-virgl branch, not wink (what the submodule tracks). Build from a" >&2
+    echo "gex-virgl checkout: GEX_MESA_SRC=/path/to/mesa-wink@gex-virgl $0" >&2
+    exit 1
+}
 
 if [ ! -f "$BUILD/build.ninja" ]; then
     mkdir -p "$OUT"