Viewport Compositor: Platform support message.

In the previous situation the message was shown for Apple devices.
But this is not correct and confusing.

- Apple with Metal backend are supported, OpenGL on Apple isn't
- Legacy devices on Windows or Linux are also not supported.

This change will check that the capabilities of the GPU match the
requirements to use Viewport compositor. Based on those capabilities
a message is shown and the panel is activated.

release/scripts/startup/bl_ui/space_view3d.py

@@ -6195,14 +6195,15 @@ class VIEW3D_PT_shading_compositor(Panel):
     def draw(self, context):
         shading = context.space_data.shading
 
-        import sys
-        is_macos = sys.platform == "darwin"
+        import gpu
+        is_supported = (gpu.capabilities.compute_shader_support_get()
+                        and gpu.capabilities.shader_image_load_store_support_get())
 
         row = self.layout.row()
-        row.active = not is_macos
+        row.active = is_supported
         row.prop(shading, "use_compositor", expand=True)
-        if is_macos and shading.use_compositor != "DISABLED":
-            self.layout.label(text="Compositor not supported on MacOS", icon='ERROR')
+        if shading.use_compositor != "DISABLED" and not is_supported:
+            self.layout.label(text="Compositor not supported on this platform", icon='ERROR')
 
 
 class VIEW3D_PT_gizmo_display(Panel):