Readability: remove redundant return, continue, enum declaration

src/audio/alsa/SDL_alsa_audio.c

@@ -340,7 +340,6 @@ swizzle_alsa_channels(_THIS, void *buffer, Uint32 bufferlen)
 static void
 no_swizzle(_THIS, void *buffer, Uint32 bufferlen)
 {
-    return;
 }
 #endif /* SND_CHMAP_API_VERSION */
 

src/joystick/SDL_gamecontroller.c

@@ -881,7 +881,7 @@ SDL_PrivateAddMappingForGUID(SDL_JoystickGUID jGUID, const char *mappingString,
             for ( pPrevMapping = s_pSupportedControllers, pCurrMapping = pPrevMapping->next;
                   pCurrMapping; 
                   pPrevMapping = pCurrMapping, pCurrMapping = pCurrMapping->next ) {
-                continue;
+                /* continue; */
             }
             pPrevMapping->next = pControllerMapping;
         } else {

src/render/opengl/SDL_render_gl.c

@@ -180,7 +180,7 @@ GL_ClearErrors(SDL_Renderer *renderer)
         }
     } else if (data->glGetError != NULL) {
         while (data->glGetError() != GL_NO_ERROR) {
-            continue;
+            /* continue; */
         }
     }
 }

src/render/opengles2/SDL_render_gles2.c

@@ -208,7 +208,7 @@ GL_ClearErrors(SDL_Renderer *renderer)
         return;
     }
     while (data->glGetError() != GL_NO_ERROR) {
-        continue;
+        /* continue; */
     }
 }
 

src/video/SDL_blit_N.c

@@ -2317,7 +2317,6 @@ get_permutation(SDL_PixelFormat *srcfmt, SDL_PixelFormat *dstfmt,
     if (_alpha_channel) {
         *_alpha_channel = alpha_channel;
     }
-    return;
 }
 
 
@@ -3109,7 +3108,6 @@ Blit_3or4_to_3or4__same_rgb(SDL_BlitInfo * info)
             dst += dstskip;
         }
     }
-    return;
 }
 
 /* Blit_3or4_to_3or4__inversed_rgb: 3 or 4 bpp, inversed RGB triplet */
@@ -3214,7 +3212,6 @@ Blit_3or4_to_3or4__inversed_rgb(SDL_BlitInfo * info)
             dst += dstskip;
         }
     }
-    return;
 }
 
 /* Normal N to N optimized blitters */

src/video/wayland/SDL_waylandtouch.c

@@ -192,7 +192,6 @@ WL_EXPORT const struct wl_interface qt_windowmanager_interface = {
 /* wayland-qt-windowmanager.c ENDS */
 
 /* wayland-qt-surface-extension.c BEGINS */
-extern const struct wl_interface qt_extended_surface_interface;
 #ifndef SDL_VIDEO_DRIVER_WAYLAND_DYNAMIC
 extern const struct wl_interface wl_surface_interface;
 #endif