[Test] (MG_IntegrationTest): pin the two shipped memo bugs with rendered pixels

Both d7976326 bugs passed every unit test while corrupting real frames -
state-level assertions cannot see them. This module renders and reads
back.

A headless EGL-pbuffer harness (no window, no GLFW) linking MobileGL_s
directly, registered once per backend under the ctest label
integration-gpu, behind the default-OFF option
MOBILEGL_BUILD_INTEGRATION_TEST. The platform pre-flight runs the ENTIRE
bring-up in a forked child first - MobileGL aborts rather than returning
errors on an unusable platform, and the child dying on any signal turns
into a clean GTEST_SKIP instead of taking the test binary down.
MOBILEGL_ITEST_REQUIRE_GPU makes the label falsifiable: with it set, an
unusable harness (or a context that lands on a software rasterizer) is a
FAILURE - without it, a CI runner whose driver pinning silently broke
reports the same green as one that rendered every frame. Configure-time
detection pins the EGL vendor and Vulkan ICD jsons, preferring hardware
vendors and never selecting llvmpipe/lavapipe.

Scenarios assert on glReadPixels with whole-region pixel counts (a
2x2 quadrant pattern whose signature distinguishes all eight square
symmetries; every region predicate reports the first offending pixel):
- OrientationScenario: default -> FBO -> default, pinning the
  transform-flags memo key. Keying GetBaseTransformFlagsRaw on the
  pre-transform alone fails exactly 3 entries.
- StreamedArenaScenario: an untouched streamed vertex buffer must
  survive transient-arena recycling. Re-enabling only the cross-frame
  vertex revalidation fails exactly this entry.
- CrossFrameBufferScenario + ResidentIndexScenario: cross-frame
  mutation matrix (SubData, map/unmap, persistent+flush, coherent
  persistent, orphan, CopyBufferSubData; vertex and index) plus six
  adversarial resident-EBO constructions. Instrumentation showed the
  cross-frame EBO memo cannot be made to serve wrong bytes from GL
  level on this stack (89 entries, 81 accepts, zero divergent slices) -
  these cases are freshness tripwires, documented as such in-file; the
  EBO half of d7976326 remains unpinned by a failing test.

At the buggy commit 72ee7c43 the suite fails 4 entries (3 orientation +
1 streamed-arena); at d7976326 all 52 pass, 5 consecutive runs, zero
flakes, and the default build is bit-for-bit unaffected (unit suite
unchanged). Adversarially verified twice, including hostile-platform
sweeps (26 configurations, all clean skips) and hand-edits of each
production hole in isolation.

CMakeLists.txt

@@ -4,6 +4,11 @@ project("MobileGL")
 
 option(MOBILEGL_BUILD_TEST        "Build MobileGL tests"                             ON )
 option(MOBILEGL_BUILD_BENCHMARK   "Build MobileGL benchmarks"                        ON )
+# Headless end-to-end GPU scenarios (MobileGL/MG_IntegrationTest). They need a
+# real GPU/ICD to do anything, so they are off by default for CI; every scenario
+# skips cleanly where there is none. Registered under the `integration-gpu`
+# ctest label so a run can select or exclude them.
+option(MOBILEGL_BUILD_INTEGRATION_TEST "Build MobileGL headless GPU integration tests" OFF)
 option(MOBILEGL_FORCE_RELEASE_OPT "Enable Release optimization flags in Debug build" ON )
 option(MOBILEGL_ENABLE_TRACY      "Enable tracy for profiling"                       OFF)
 option(MOBILEGL_BUILD_TRACE_REPLAY "Build desktop apitrace replay runner"            OFF)
@@ -538,6 +543,12 @@ if (NOT ANDROID)
         add_subdirectory(MobileGL/MG_Test)
     endif()
 
+    # After MG_Test so googletest is already available when the unit tests are
+    # built; the module fetches its own copy when they are not.
+    if (MOBILEGL_BUILD_INTEGRATION_TEST)
+        add_subdirectory(MobileGL/MG_IntegrationTest)
+    endif()
+
     if (MOBILEGL_BUILD_BENCHMARK)
         add_subdirectory(MobileGL/MG_Benchmark)
     endif()