Added MultipleWindows platform capability.

Several platform plugins, like eglfs, kms, etc don't support multiple
windows as there's no system compositor, they're rendering directly to
a single back buffer. By adding a platform capability we'll be able to
provide better error reporting when an application tries to create
multiple QWindows on a single-window platform. Also, QML apps can use
this capability to figure out whether they should create a QWindow for
dialogs / popups / menus, or whether to just create items in the same
scene, that are shown on top of the rest of the content.

Change-Id: I15b8d21ee2bc4568e9d705dbf32f872c2c25742b
Reviewed-by: Andy Nichols <andy.nichols@digia.com>

src/gui/kernel/qplatformintegration.cpp

@@ -195,6 +195,10 @@ QPlatformServices *QPlatformIntegration::services() const
     \value BufferQueueingOpenGL The OpenGL implementation on the platform will queue
     up buffers when swapBuffers() is called and block only when its buffer pipeline
     is full, rather than block immediately.
+
+    \value MultipleWindows The platform supports multiple QWindows, i.e. does some kind
+    of compositing either client or server side. Some platforms might only support a
+    single fullscreen window.
  */
 
 

src/gui/kernel/qplatformintegration.h

@@ -83,11 +83,12 @@ class Q_GUI_EXPORT QPlatformIntegration
 public:
     enum Capability {
         ThreadedPixmaps = 1,
-        OpenGL = 2,
+        OpenGL,
-        ThreadedOpenGL = 3,
+        ThreadedOpenGL,
-        SharedGraphicsCache = 4,
+        SharedGraphicsCache,
-        BufferQueueingOpenGL = 5,
+        BufferQueueingOpenGL,
-        WindowMasks = 6
+        WindowMasks,
+        MultipleWindows
     };
 
     virtual ~QPlatformIntegration() { }

src/plugins/platforms/cocoa/qcocoaintegration.mm

@@ -309,6 +309,7 @@ bool QCocoaIntegration::hasCapability(QPlatformIntegration::Capability cap) cons
     case ThreadedOpenGL:
     case BufferQueueingOpenGL:
     case WindowMasks:
+    case MultipleWindows:
         return true;
     default:
         return QPlatformIntegration::hasCapability(cap);

src/plugins/platforms/minimal/qminimalintegration.cpp

@@ -74,6 +74,7 @@ bool QMinimalIntegration::hasCapability(QPlatformIntegration::Capability cap) co
 {
     switch (cap) {
     case ThreadedPixmaps: return true;
+    case MultipleWindows: return true;
     default: return QPlatformIntegration::hasCapability(cap);
     }
 }

src/plugins/platforms/windows/qwindowsintegration.cpp

@@ -338,6 +338,8 @@ bool QWindowsIntegration::hasCapability(QPlatformIntegration::Capability cap) co
 #endif // !QT_NO_OPENGL
     case WindowMasks:
         return true;
+    case MultipleWindows:
+        return true;
     default:
         return QPlatformIntegration::hasCapability(cap);
     }

src/plugins/platforms/xcb/qxcbintegration.cpp

@@ -216,6 +216,7 @@ bool QXcbIntegration::hasCapability(QPlatformIntegration::Capability cap) const
 #endif
     case ThreadedOpenGL: return false;
     case WindowMasks: return true;
+    case MultipleWindows: return true;
     default: return QPlatformIntegration::hasCapability(cap);
     }
 }