[Feat] (MG_Impl, MG_Backend, MG_State): implement glMultiDrawArrays and glGetBufferSubData

Two previously-stubbed GL 3.3 Core entry points.

glMultiDrawArrays: mirrors the existing glMultiDrawElements(BaseVertex)
architecture end to end -- a new MultiDrawArrays backend function-table
slot dispatched from the frontend after program/primitive-mode validation
(plus a drawcount < 0 -> GL_INVALID_VALUE guard).
- DirectGLES: PrepareForDraw once, then loop native glDrawArrays with the
  same per-range client-side array upload the single DrawArrays does.
- DirectVulkan: build a MultiDrawCmd payload and hand it to a new
  VulkanRenderer::MultiDrawArrays, which does one SetupDraw over the union
  of the sub-draw vertex ranges and then a vkCmdDraw per range (mirrors
  VulkanRenderer::MultiDrawElements).

glGetBufferSubData: reads a range of the bound buffer's CPU shadow into
client memory via a new BufferObject::DownloadSubData, with the same
validation shape as BufferSubData (INVALID_VALUE for negative/overflowing
range, INVALID_OPERATION for no bound buffer or a non-persistent mapped
buffer). The shadow reflects CPU writes and backend write-backs but not
arbitrary GPU-side writes, which is documented on the method.

Tests: 2 BufferTest cases for glGetBufferSubData (round-trip read of a
middle range and the whole buffer, plus out-of-range/negative/no-buffer
errors). BufferTest 32/32, SanityTest 30/30, VertexArrayTest 42/42;
library builds clean. (The glMultiDrawArrays draw paths are not
runtime-testable on this host and are compile-verified against the tested
MultiDrawElements pattern.)

MobileGL/MG_State/GLState/BufferState/BufferObject.cpp

@@ -185,6 +185,13 @@ namespace MobileGL::MG_State::GLState {
         NotifySubData(atOffset, data.size);
     }
 
+    void BufferObject::DownloadSubData(void* dst, SizeT atOffset, SizeT size) const {
+        MOBILEGL_ASSERT(atOffset + size <= m_size,
+                        "DownloadSubData out of bounds: atOffset (%zu) + size (%zu) > m_size (%zu)", atOffset, size,
+                        m_size);
+        Memcpy(dst, m_dataPtr->data() + atOffset, size);
+    }
+
     void BufferObject::CopyDataFrom(const SharedPtr<BufferObject>& src, SizeT srcOffset, SizeT dstOffset, SizeT size) {
         MOBILEGL_ASSERT(!m_isMapped || (m_mappingAccess & BufferMappingAccessBit::Persistent),
                         "Cannot copy data while destination buffer is non-persistently mapped.");