Instead of `gdi_fabulous` introduced some bit flags for enabling/disabling features individually. Mainly for testing and benchmarking purposes of these features until we decide the best implementation for each of them. So they are enabled by setting flags with `Clay_Win32_SetRendererFlags()` in user code (demo in this case)
At first I liked the idea to use something more semantic to pass as `HWND` to get the screen device context. And there is a HWND_DESKTOP macro which just a simple alias to NULL, but it is actually designed for CreateWindow and naming isn't clear enough. It may confuse someone who reading code that we're trying to get not the screen dc, but that desktop window dc itself. So I won't stand out let it be NULL as that commonly used.
The implementation works by handling special cases where rectangles have semi-transparent background colors or rounded corners. For these cases, it first copies the underlying image region from the window device context, blends the rectangle on top of this captured background, and then draws the final result back to the window DC because we can't access and modify window dc bits directly. Also this approach requires custom pixel processing for color blending and uses sqrtf for precise anti-aliased corners, which isn't ideal for performance. A `gdi_fabulous` flag is provided to toggle this feature when needed.
