Fix DRM EGL crash regression

Summary:
In 47343fb we made GBM buffer shared.

What we wanted to do was:
Unbox the shared_pointer<GBMSurface> to give us a GBMSurface* object
Call the gbm_surface*() on that operator
Then cast that to a void* for eglCreatePlatformWindowSurfaceEXT

What we did:
Cast the std::shared_ptr<GBMSurface> to a gbm_surface*  then cast that
to void*.
This is just a garbage value and it crashes in Mesa when we do our first
paint.

I've replaced that with an explicit method then we can use shared_ptr's
-> operator rather than get() which does the right thing in a readable
way.

Test Plan:
It crashed after rebasing to master (for Aleix too)
No longer crashes

Reviewers: #plasma

Subscribers: plasma-devel, kwin, #kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D8251

autotests/test_gbm_surface.cpp

@@ -78,7 +78,7 @@ private Q_SLOTS:
 void GbmSurfaceTest::testCreate()
 {
     GbmSurface surface(nullptr, 2, 3, 4, 5);
-    gbm_surface *native = surface;
+    gbm_surface *native = surface.surface();
     QVERIFY(surface);
     QCOMPARE(native->width, 2u);
     QCOMPARE(native->height, 3u);
@@ -91,7 +91,7 @@ void GbmSurfaceTest::testCreateFailure()
     gbm_device dev{true};
     GbmSurface surface(&dev, 2, 3, 4, 5);
     QVERIFY(!surface);
-    gbm_surface *native = surface;
+    gbm_surface *native = surface.surface();
     QVERIFY(!native);
 }
 

plugins/platforms/drm/egl_gbm_backend.cpp

@@ -162,7 +162,7 @@ void EglGbmBackend::createOutput(DrmOutput *drmOutput)
         qCCritical(KWIN_DRM) << "Create gbm surface failed";
         return;
     }
-    o.eglSurface = eglCreatePlatformWindowSurfaceEXT(eglDisplay(), config(), (void *)((gbm_surface*)o.gbmSurface.get()), nullptr);
+    o.eglSurface = eglCreatePlatformWindowSurfaceEXT(eglDisplay(), config(), (void *)(o.gbmSurface->surface()), nullptr);
     if (o.eglSurface == EGL_NO_SURFACE) {
         qCCritical(KWIN_DRM) << "Create Window Surface failed";
         o.gbmSurface.reset();

plugins/platforms/drm/gbm_surface.h

@@ -42,7 +42,7 @@ public:
         return m_surface != nullptr;
     }
 
-    operator gbm_surface*() const {
+    gbm_surface* surface() const {
         return m_surface;
     }