Different blending compared to official OpenGL backend

<!--Thank you for filing this! Please provide as much information as possible to help us diagnose and fix the issue--> |OpenGL|Wgpu| |---|---| ||| ## Description The alpha blending looks different compared to the ImGui OpenGL 3 backend. ## Repro steps <!--A runable example or short code sample showing how to reproduce the bug--> OpenGL is based on `example_glfw_opengl3`, Wgpu is based on `imgui-wgpu-rs` example. ## System Information <!--Version of imgui-wgpu-rs, information about your OS if relevant, your tech stack, etc.--> - `imgui-wgpu-rs` 0.20.0 - Arch Linux, 6.0.2 - Wayland, Sway - AMD Radeon iGPU