Simplify size changes in fullscreen or maximized

If the content size changes (due to rotation for example) while the
window is maximized or fullscreen, the resize must be applied once
fullscreen and maximized are disabled.

The previous strategy consisted in storing the windowed size, computing
the target size on rotation, and applying it on window restoration. But
tracking the windowed size (while ignoring the non-windowed size) was
tricky, due to unspecified order of SDL events (e.g. size changes can be
notified before "maximized" events), race conditions when reading window
flags, different behaviors on different platforms...

To simplify the whole resize management, store the old content size (the
frame size, possibly rotated) when it changes while the window is
maximized or fullscreen, so that the new optimal size can be computed on
window restoration.

app/src/screen.h

@@ -21,11 +21,12 @@ struct screen {
     struct sc_opengl gl;
     struct size frame_size;
     struct size content_size; // rotated frame_size
-    // The window size the last time it was not maximized or fullscreen.
-    struct size windowed_window_size;
-    // Since we receive the event SIZE_CHANGED before MAXIMIZED, we must be
-    // able to revert the size to its non-maximized value.
-    struct size windowed_window_size_backup;
+
+    bool resize_pending; // resize requested while fullscreen or maximized
+    // The content size the last time the window was not maximized or
+    // fullscreen (meaningful only when resize_pending is true)
+    struct size windowed_content_size;
+
     // client rotation: 0, 1, 2 or 3 (x90 degrees counterclockwise)
     unsigned rotation;
     bool has_frame;
@@ -49,11 +50,8 @@ struct screen {
         .width = 0, \
         .height = 0, \
     }, \
-    .windowed_window_size = { \
-        .width = 0, \
-        .height = 0, \
-    }, \
-    .windowed_window_size_backup = { \
+    .resize_pending = false, \
+    .windowed_content_size = { \
         .width = 0, \
         .height = 0, \
     }, \