camera: Massive code reworking.

- Simplified public API, simplified backend interface.
- Camera device hotplug events.
- Thread code is split up so it backends that provide own threads can use it.
- Added "dummy" backend.

Note that CoreMedia (Apple) and Android backends need to be updated, as does
the testcamera app (testcameraminimal works).

test/testcamera.c

@@ -18,8 +18,13 @@
 #include <emscripten/emscripten.h>
 #endif
 
-#include <stdio.h>
-
+#if 1
+int main(int argc, char **argv)
+{
+    SDL_Log("FIXME: update me");
+    return 0;
+}
+#else
 static const char *usage = "\
  \n\
  =========================================================================\n\
@@ -769,3 +774,4 @@ int main(int argc, char **argv)
 
     return 0;
 }
+#endif