Android: fix call of glFinish without context.

Message in the log, when going to background:
"call to OpenGL ES API with no current context (logged once per thread)"

Because of SDL_WINDOWEVENT_MINIMIZED is sent from the Java Activity thread.
It calls SDL_RendererEventWatch(), _WindowEvent() and glFinish() without context.

Solution is to move sending of SDL_WINDOWEVENT_MINIMIZED to the SDL thread.

src/core/android/SDL_android.c

@@ -1162,23 +1162,11 @@ JNIEXPORT void JNICALL SDL_JAVA_INTERFACE(nativeQuit)(
 JNIEXPORT void JNICALL SDL_JAVA_INTERFACE(nativePause)(
                                     JNIEnv *env, jclass cls)
 {
-    SDL_LockMutex(Android_ActivityMutex);
-
     __android_log_print(ANDROID_LOG_VERBOSE, "SDL", "nativePause()");
 
-    if (Android_Window) {
-        SDL_SendWindowEvent(Android_Window, SDL_WINDOWEVENT_MINIMIZED, 0, 0);
-        SDL_SendAppEvent(SDL_APP_WILLENTERBACKGROUND);
-        SDL_SendAppEvent(SDL_APP_DIDENTERBACKGROUND);
-    }
-
-    /* *After* sending the relevant events, signal the pause semaphore
-     * so the event loop knows to pause and (optionally) block itself.
-     * Sometimes 2 pauses can be queued (eg pause/resume/pause), so it's
-     * always increased. */
+    /* Signal the pause semaphore so the event loop knows to pause and (optionally) block itself.
+     * Sometimes 2 pauses can be queued (eg pause/resume/pause), so it's always increased. */
     SDL_SemPost(Android_PauseSem);
-
-    SDL_UnlockMutex(Android_ActivityMutex);
 }
 
 /* Resume */

src/video/android/SDL_androidevents.c

@@ -27,6 +27,7 @@
 #include "SDL_androidkeyboard.h"
 #include "SDL_androidwindow.h"
 #include "../SDL_sysvideo.h"
+#include "../../events/SDL_events_c.h"
 
 /* Can't include sysaudio "../../audio/android/SDL_androidaudio.h"
  * because of THIS redefinition */
@@ -129,6 +130,14 @@ Android_PumpEvents_Blocking(_THIS)
         }
     } else {
         if (videodata->isPausing || SDL_SemTryWait(Android_PauseSem) == 0) {
+
+            /* Android_PauseSem was signaled */
+            if (videodata->isPausing == 0) {
+                SDL_SendWindowEvent(Android_Window, SDL_WINDOWEVENT_MINIMIZED, 0, 0);
+                SDL_SendAppEvent(SDL_APP_WILLENTERBACKGROUND);
+                SDL_SendAppEvent(SDL_APP_DIDENTERBACKGROUND);
+            }
+
             /* We've been signaled to pause (potentially several times), but before we block ourselves,
              * we need to make sure that the very last event (of the first pause sequence, if several)
              * has reached the app */
@@ -187,6 +196,14 @@ Android_PumpEvents_NonBlocking(_THIS)
         }
     } else {
         if (videodata->isPausing || SDL_SemTryWait(Android_PauseSem) == 0) {
+
+            /* Android_PauseSem was signaled */
+            if (videodata->isPausing == 0) {
+                SDL_SendWindowEvent(Android_Window, SDL_WINDOWEVENT_MINIMIZED, 0, 0);
+                SDL_SendAppEvent(SDL_APP_WILLENTERBACKGROUND);
+                SDL_SendAppEvent(SDL_APP_DIDENTERBACKGROUND);
+            }
+
             /* We've been signaled to pause (potentially several times), but before we block ourselves,
              * we need to make sure that the very last event (of the first pause sequence, if several)
              * has reached the app */