[Feat] (DirectVulkan): emulate GL_LINE_LOOP with closed indexed line strips

Vulkan has no LINE_LOOP topology and the frontend used to reject the mode
with GL_INVALID_OPERATION, which is itself non-conformant (several KHR-GL33
transform_feedback tests draw line loops and expect no error). DrawArrays,
DrawElements and DrawElementsBaseVertex now rewrite the draw into an
indexed GL_LINE_STRIP whose synthesized uint32 index list revisits the
first vertex, delivered through the client-memory index path (a new
forceClientMemory flag keeps a bound element-array buffer from hijacking
the synthesized pointer). Entry points without the rewrite degrade to an
open line strip instead of a triangle list.

MobileGL/MG_Backend/DirectVulkan/Renderer/VulkanRenderer.h

@@ -76,6 +76,10 @@ namespace MobileGL::MG_Backend::DirectVulkan {
         GLenum indexType = GL_UNSIGNED_SHORT;
         SizeT indexByteOffset = 0;
         SizeT indexByteSize = 0;
+        // Interpret indexByteOffset as a raw client pointer even when an element
+        // array buffer is bound (backend-synthesized index lists, e.g. the
+        // GL_LINE_LOOP -> LINE_STRIP rewrite).
+        Bool forceClientMemory = false;
     };
 
     struct DrawIndexedCmd {