Backends: Allegro5: Brough back al_draw_indexed_prim when available. (#5937)

Amend 185b4dde

backends/imgui_impl_allegro5.cpp

@@ -7,7 +7,7 @@
 //  [X] Platform: Clipboard support (from Allegro 5.1.12)
 //  [X] Platform: Mouse cursor shape and visibility. Disable with 'io.ConfigFlags |= ImGuiConfigFlags_NoMouseCursorChange'.
 // Issues:
-//  [ ] Renderer: The renderer is suboptimal as we need to unindex our buffers and convert vertices manually.
+//  [ ] Renderer: The renderer is suboptimal as we need to convert vertices manually.
 //  [ ] Platform: Missing gamepad support.
 
 // You can use unmodified imgui_impl_* files in your project. See examples/ folder for examples of using this.
@@ -132,13 +132,12 @@ void ImGui_ImplAllegro5_RenderDrawData(ImDrawData* draw_data)
     {
         const ImDrawList* cmd_list = draw_data->CmdLists[n];
 
-        // Allegro's implementation of al_draw_indexed_prim() for DX9 is completely broken. Unindex our buffers ourselves.
         // FIXME-OPT: Unfortunately Allegro doesn't support 32-bit packed colors so we have to convert them to 4 float as well..
         static ImVector<ImDrawVertAllegro> vertices;
-        vertices.resize(cmd_list->IdxBuffer.Size);
-        for (int i = 0; i < cmd_list->IdxBuffer.Size; i++)
+        vertices.resize(cmd_list->VtxBuffer.Size);
+        for (int i = 0; i < cmd_list->VtxBuffer.Size; i++)
         {
-            const ImDrawVert* src_v = &cmd_list->VtxBuffer[cmd_list->IdxBuffer[i]];
+            const ImDrawVert* src_v = &cmd_list->VtxBuffer[i];
             ImDrawVertAllegro* dst_v = &vertices[i];
             dst_v->pos = src_v->pos;
             dst_v->uv = src_v->uv;
@@ -187,7 +186,9 @@ void ImGui_ImplAllegro5_RenderDrawData(ImDrawData* draw_data)
                 // Apply scissor/clipping rectangle, Draw
                 ALLEGRO_BITMAP* texture = (ALLEGRO_BITMAP*)pcmd->GetTexID();
                 al_set_clipping_rectangle(clip_min.x, clip_min.y, clip_max.x - clip_min.x, clip_max.y - clip_min.y);
-                al_draw_prim(&vertices[0], bd->VertexDecl, texture, pcmd->IdxOffset, pcmd->IdxOffset + pcmd->ElemCount, ALLEGRO_PRIM_TRIANGLE_LIST);
+                if(pcmd->ElemCount > 0) {
+                    al_draw_indexed_prim(&vertices[0], bd->VertexDecl, texture, &indices[pcmd->IdxOffset], pcmd->ElemCount, ALLEGRO_PRIM_TRIANGLE_LIST);
+                }
             }
         }
     }