camera: Emscripten support!

This also adds code to deal with waiting for the user to approve camera
access, reworks testcameraminimal to use main callbacks, etc.

src/dynapi/SDL_dynapi.sym

@@ -970,6 +970,7 @@ SDL3_0.0.0 {
     SDL_AcquireCameraFrame;
     SDL_ReleaseCameraFrame;
     SDL_CloseCamera;
+    SDL_GetCameraPermissionState;
     # extra symbols go here (don't modify this line)
   local: *;
 };