[Feat] (MG_Backend): port MobileGlues' multi-draw emulation to DirectGLES as a tier ladder

ES has neither glMultiDrawElements nor glMultiDrawElementsBaseVertex, so
both are emulated. DirectGLES had two ways of doing it - one
glMultiDrawElementsBaseVertexEXT where the driver has the extension
interaction, otherwise a per-draw loop. This adds the five MobileGlues
uses (gl/multidraw.cpp), so the ladder is now: one
glMultiDrawElementsBaseVertexEXT; one glMultiDrawElementsIndirectEXT
over a synthesized command buffer; one glDrawElementsIndirect per
command over that same buffer; the base-vertex replay; plain
glDrawElements over a CPU-rewritten index stream, for drivers with no
base-vertex draw at all; and a compute shader that flattens the whole
batch into one rebased index buffer drawn by a single glDrawElements.
They live in their own translation unit that owns the entry point
outright, preparation included - the compute tier has to dispatch BEFORE
PrepareForDraw, or it would have to unpick the program, storage-block
and index bindings the preparation just made, and a dispatch inside an
open transform-feedback span is not legal at all.

The auto ladder is ext -> basevertex -> multiindirect -> indirect ->
drawelements, which is NOT MobileGlues' order (it puts the indirect
tiers first). Measured on mc_sodium_multidraw, ns/op, median of three:
NVIDIA ES 3.2 basevertex 2500 vs multiindirect 5700 and indirect 5800;
Mesa llvmpipe ext 19300, basevertex 25200, multiindirect 27600,
drawelements 28700, indirect 31000. Ring-allocating the command staging
instead of respecifying per batch was tried first and moved the indirect
tiers by less than noise, so the cost is the indirect draw path itself,
not the upload; only a real multi-draw entry point beats replaying the
sub-draws. auto therefore resolves to basevertex on this box - byte for
byte the behaviour that shipped - and the new tiers are what a driver
with the ext interaction, or without base vertex at all, now gets.
compute is never chosen by auto (nor by MobileGlues'): it rewrites the
primitive stream rather than replaying it, and it measured slowest here.

Four places this deliberately does not follow MobileGlues, each a
correctness bug there. A rewritten stream is emitted as GL_UNSIGNED_INT
whatever came in, because GL adds baseVertex at full precision and
folding it into ushort indices wraps. The restart sentinel is carried
across a rebase unrebased, or an enabled primitive restart is lost. The
flattening tier declines strip/loop/fan modes, any sub-draw whose count
is not a whole number of primitives, and any batch at all while
primitive restart is enabled - a restart ends a primitive, so leftover
vertices would find a third vertex in the next sub-draw and become a
triangle GL never draws. And the indirect tiers decline client-memory
index arrays, which have no buffer to address.

gl_DrawID gets better rather than worse: the unrolled tiers now feed
each sub-draw its index (the spec's value, where the old loop left the
uniform untouched), and a program that actually reads it demotes the
batched tiers, which can only hold one value for the whole batch. The
per-batch cost is nil for the programs that do not read it.

Verified: the five DirectGLES retraces are byte-identical (md5) across
all six tiers on NVIDIA and on Mesa, each tier proven to have really
executed rather than silently demoted, via a per-tier announcement in
the log. Unit suite 421/421. The full retrace suite's five failures all
reproduce unchanged on a stashed tree, so none are new.

MobileGL/MG_Backend/DirectGLES/DirectGLES.h

@@ -182,6 +182,11 @@ namespace MobileGL::MG_Backend::DirectGLES {
 
     namespace XfbImpl {
         Bool AreTransformFeedbacksSupported();
+        // True while a capture span is open on the current transform feedback object
+        // (frontend Begin seen and not paused), whether or not the deferred driver-side
+        // Begin has been issued yet. Draw paths that would restructure the primitive
+        // stream, or that need to dispatch compute mid-draw, decline while it is set.
+        Bool IsCaptureSpanOpen();
         void BeginTransformFeedback(GLenum primitiveMode);
         void EndTransformFeedback();
         void PauseTransformFeedback();