[Feat] (MG_Util, MG_IntegrationTest): POST rows for the Espryt multi-draw tier, and the scenario that pins it

Three DriverPost rows per the POST rule, since the ladder took on two
new driver dependencies: glDrawElementsBaseVertex (WARN when absent -
every base-vertex draw then costs a CPU index rewrite and an upload) and
compute shaders (INFO - the default tiers never use them). The third
names the tier that will actually run, with the full set the driver
supports, resolved by the same function the backend calls so the two can
not drift. The existing "Multi-draw base vertex" row stopped saying the
fallback is a per-draw loop, which is no longer the whole truth.

Scenario D asserts the one contract every tier shares: a multi-draw
paints exactly what the unrolled single draws paint. The reference side
is a loop of glDrawElementsBaseVertex and never enters the emulation, so
a tier cannot make itself look right by breaking both sides alike, and a
blank-frame pair is rejected outright - drawing nothing is the failure
mode this path actually has.

Nine cases, chosen for the shapes the Minecraft retraces contain none
of: narrow index types, a base vertex past the index type's range,
primitive restart inside a strip on two index types, client-memory index
arrays, and a batch with zero-count sub-draws (whose prefix sums the
flattening tier's binary search has to skip). Each of the six tiers
passes all nine on NVIDIA, and ext/auto/compute also pass on Mesa where
the ext tier is reachable.

The suite is falsifiable, not merely green: rewriting the rebase the way
MobileGlues does it - truncate to the source width, no restart
passthrough - fails exactly three cases on the drawelements tier (both
restart cases and the out-of-range base vertex) and leaves basevertex,
which rewrites nothing, passing. That control is also what turned up the
restart hole in the flattening tier fixed in the previous commit.

MobileGL/MG_IntegrationTest/CMakeLists.txt

@@ -49,6 +49,7 @@ add_executable(MobileGLIntegrationTest
     Scenarios/OrientationScenario.cpp
     Scenarios/CrossFrameBufferScenario.cpp
     Scenarios/ResidentIndexScenario.cpp
+    Scenarios/MultiDrawScenario.cpp
 )
 
 target_include_directories(MobileGLIntegrationTest PRIVATE