Add EMU_GPU=software, for work where the GPU is not what is measured

`-gpu host` runs the guest's GLES on this machine's real GPU, which here is
the host's, reached through virtio-gpu. That is right for the Compose
scroll benchmarks and was measured -- 25% janky frames against 3%, and
3.9 GB resident against 2.9 GB -- so it stays the default and nothing any
other session runs changes.

EMU_GPU=software takes the software branch that already existed for the
no-compositor case, for work where the emulator's own frame rate is not
under test. Two things it buys: no GPU use at all, and a guest Vulkan
device (SwiftShader, verified by `cmd gpu vkjson`), which is otherwise
unavailable here because gfxstream's opaque-fd external memory cannot pair
with Venus. Measured while it booted: qemu at about 490% CPU and a load
average of 5, with io pressure at zero.

The condition is on the existing branches rather than a fourth one, so
there is still one place that says how software rendering is spelled.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

bin/emu

@@ -49,9 +49,10 @@ usage: emu <command>
   check    refuse if a command that reaches every attached device would
            reach another checkout's emulator from here
 
-Environment: AVD_NAME overrides the name, EMU_FORCE=1 overrides the memory
+Environment: EMU_GPU=software renders on the CPU instead of the host's GPU,
-refusal, EMU_ANY_DEVICE=1 overrides `check`, DEVICE_PROFILE and SYSTEM_IMAGE
+AVD_NAME overrides the name, EMU_FORCE=1 overrides the memory refusal,
-decide what `up` creates.
+EMU_ANY_DEVICE=1 overrides `check`, DEVICE_PROFILE and SYSTEM_IMAGE decide
+what `up` creates.
 USAGE
     exit 2
 }
@@ -225,15 +226,29 @@ cmd_up() {
 
     log="/tmp/$avd-emulator.log"
     rm -f "$log"
-    if [ -n "${DISPLAY:-}" ] || [ -n "${WAYLAND_DISPLAY:-}" ]; then
+    # `-gpu host` runs the guest's GLES on this machine's real GPU, which here
+    # means the *host's* GPU reached through virtio-gpu -- so a headless
+    # emulator nobody is watching still renders onto the desktop somebody is
+    # sitting at. It stays the default because it was measured (25% janky
+    # frames against 3%, and 3.9 GB resident against 2.9 GB) and the Compose
+    # scroll benchmarks depend on it. EMU_GPU=software opts out, for work
+    # where the emulator's own frame rate is not what is under test: it
+    # renders on the CPU and touches no GPU at all, which also means the
+    # guest gets a software Vulkan device (SwiftShader) instead of the host
+    # Vulkan that gfxstream cannot pair with Venus here.
+    if [ "${EMU_GPU:-}" != software ] && { [ -n "${DISPLAY:-}" ] || [ -n "${WAYLAND_DISPLAY:-}" ]; }; then
         echo "emu: starting '$avd' with GPU acceleration" >&2
         "$sdk/emulator/emulator" -avd "$avd" -gpu host -no-audio >"$log" 2>&1 &
-    elif display=$(headless_display); then
+    elif [ "${EMU_GPU:-}" != software ] && display=$(headless_display); then
         echo "emu: no display -- starting '$avd' headless on the GPU ($display)" >&2
         DISPLAY="$display" "$sdk/emulator/emulator" -avd "$avd" \
             -gpu host $GPU_HOST_FEATURES -no-audio -no-window >"$log" 2>&1 &
+    else
+        if [ "${EMU_GPU:-}" = software ]; then
+            echo "emu: EMU_GPU=software -- starting '$avd' on the CPU, off the host GPU" >&2
         else
             echo "emu: no display and no compositor -- starting '$avd' on software rendering" >&2
+        fi
         "$sdk/emulator/emulator" -avd "$avd" -gpu swiftshader_indirect -no-audio -no-window \
             >"$log" 2>&1 &
     fi