libobs: Fix clear color mismatch in Metal and OpenGL on Apple Silicon

The OpenGL frontend is implemented in Metal for Apple Silicon Macs.

Metal does not provide a direct "clear" command (neither do D3D12 or
Vulkan) and instead a clear is a load command for a render target of a
render pass.

All writes (and color calculations) in Metal happen in linear space,
with appropriate gamma encoding and decoding happening automatically
depending on the pixel format of the the textures attached to the
pipeline.

Thus setting a clear color with linear gamma is the correct approach to
avoid the SRGB gamma curve being applied twice for both scenarios.

libobs/obs-display.c

@@ -27,9 +27,6 @@ bool obs_display_init(struct obs_display *display, const struct gs_init_data *gr
 #if defined(_WIN32)
 	/* Conservative test for NVIDIA flickering in multi-GPU setups */
 	display->use_clear_workaround = gs_get_adapter_count() > 1 && !gs_can_adapter_fast_clear();
-#elif defined(__APPLE__)
-	/* Apple Silicon GL driver doesn't seem to track SRGB clears correctly */
-	display->use_clear_workaround = true;
 #else
 	display->use_clear_workaround = false;
 #endif
@@ -188,10 +185,23 @@ static inline bool render_display_begin(struct obs_display *display, uint32_t cx
 	if (success) {
 		gs_begin_scene();
 
+		/*
+		 * In contrast to OpenGL or Direct3D 11, Metal and Direct3D 12 require the clear color to use linear gamma
+		 * as either the load command to clear the render target (Metal) or the explicit clear command seem to operate
+		 * on the render target in linear space.
+		 *
+		 * As OpenGL is implemented via Metal on Apple Silicon Macs and "glClear" has to be emulated via an explicit
+		 * render pass that returns the clear color for every fragment, the color becomes subject to automatic sRGB
+		 * gamma encoding if the render target uses an sRGB color format.
+		 */
+#if defined(__APPLE__) && defined(__aarch64__)
+		vec4_from_rgba_srgb(&clear_color, display->background_color);
+#else
 		if (gs_get_color_space() == GS_CS_SRGB)
 			vec4_from_rgba(&clear_color, display->background_color);
 		else
 			vec4_from_rgba_srgb(&clear_color, display->background_color);
+#endif
 		clear_color.w = 1.0f;
 
 		const bool use_clear_workaround = display->use_clear_workaround;