Backends: DX12: Fix D3D12 Debug Layer warning if scissor rect is 0 width or 0 height. (#3472, #3462)

In the event where the scissor rect is 0 width or 0 height, don't call Draw, as it generates warnings if the D3D12 Debug Layer is enabled, and nothing would have been drawn anyway.

docs/CHANGELOG.txt

@@ -70,6 +70,7 @@ Other Changes:
   own render pass. (#3455, #3459) [@FunMiles]
 - Backends: DX12: Clarified that imgui_impl_dx12 can be compiled on 32-bit systems by redefining
   the ImTextureID to be 64-bit (e.g. '#define ImTextureID ImU64' in imconfig.h). (#301)
+- Backends: DX12: Fix debug layer warning when scissor rect is zero-sized. (#3472, #3462) [@StoneWolf]
 - Examples: Vulkan: Reworked buffer resize handling, fix for Linux/X11. (#3390, #2626) [@RoryO]
 - Examples: Vulkan: Switch validation layer to use "VK_LAYER_KHRONOS_validation" instead of
   "VK_LAYER_LUNARG_standard_validation" which is deprecated (#3459) [@FunMiles]

examples/imgui_impl_dx12.cpp

@@ -235,11 +235,14 @@ void ImGui_ImplDX12_RenderDrawData(ImDrawData* draw_data, ID3D12GraphicsCommandL
             {
                 // Apply Scissor, Bind texture, Draw
                 const D3D12_RECT r = { (LONG)(pcmd->ClipRect.x - clip_off.x), (LONG)(pcmd->ClipRect.y - clip_off.y), (LONG)(pcmd->ClipRect.z - clip_off.x), (LONG)(pcmd->ClipRect.w - clip_off.y) };
+                if (r.right > r.left && r.bottom > r.top)
+                {
                     ctx->SetGraphicsRootDescriptorTable(1, *(D3D12_GPU_DESCRIPTOR_HANDLE*)&pcmd->TextureId);
                     ctx->RSSetScissorRects(1, &r);
                     ctx->DrawIndexedInstanced(pcmd->ElemCount, 1, pcmd->IdxOffset + global_idx_offset, pcmd->VtxOffset + global_vtx_offset, 0);
                 }
             }
+        }
         global_idx_offset += cmd_list->IdxBuffer.Size;
         global_vtx_offset += cmd_list->VtxBuffer.Size;
     }