[Fix] (MG_Backend/DirectGLES): rebase gl_InstanceID for native indirect draws on ANGLE

ES keeps gl_InstanceID zero-based and ignores the indirect command's
'reserved, must be zero' word, but ANGLE-on-Vulkan forwards the command
verbatim to vkCmdDraw*Indirect and compiles gl_InstanceID to SPIR-V
InstanceIndex, which includes firstInstance. Shaders computing
gl_BaseInstance + gl_InstanceID (Flywheel indirect) then add the base
twice, scrambling instance-to-mesh association.

Probe the actual driver semantics at capability-fill time with a tiny
indirect draw (an ES indirect draw needs a non-default VAO) and, on
leaking drivers, rewrite vertex shaders that use the native indirect
SSBO machinery so gl_InstanceID subtracts the command's baseInstance
word during native indirect draws.

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

MobileGL/MG_Backend/DirectGLES/DirectGLES.cpp

@@ -1078,8 +1078,11 @@ namespace MobileGL::MG_Backend::DirectGLES {
     // shader emulation. Without GL_EXT_base_instance a non-zero baseInstance in the command is
     // technically undefined in ES; mobile drivers ignore the reserved word, instanced-array
     // fetches were never baseInstance-offset here anyway, and the CPU fallback cannot see
-    // GPU-written command fields at all - so native is never worse. Only client-memory
-    // commands take the CPU per-command loop.
+    // GPU-written command fields at all - so native is never worse. ANGLE-on-Vulkan instead
+    // leaks the word into gl_InstanceID (it becomes vkCmdDrawIndexedIndirect's firstInstance);
+    // the shader rewrite compensates by rebasing gl_InstanceID during these draws when
+    // IndirectDrawInstanceIdIncludesBaseInstance is set (PromoteDrawParameterGlobalsToUniforms).
+    // Only client-memory commands take the CPU per-command loop.
     static void ExecuteIndexedIndirectCommands(GLenum mode, GLenum type, SizeT indexSize, const Uint8* commandBytes,
                                                SizeT commandOffset,
                                                const SharedPtr<MG_State::GLState::BufferObject>& drawIndirectBuffer,