[plugins/hwcomposer] Use an RGBA_8888 format for rendering surface

Summary:
When sharing surface and config used by the compositor scene with
QtQuick's OpenGL context an RGBA surface makes Qt way more happy.

Reviewers: #kwin, #plasma_on_wayland

Subscribers: plasma-devel, kwin

Tags: #plasma_on_wayland, #kwin

Differential Revision: https://phabricator.kde.org/D2232

plugins/platforms/hwcomposer/egl_hwcomposer_backend.cpp

@@ -75,7 +75,7 @@ bool EglHwcomposerBackend::initBufferConfigs()
         EGL_RED_SIZE,             8,
         EGL_GREEN_SIZE,           8,
         EGL_BLUE_SIZE,            8,
-        EGL_ALPHA_SIZE,           0,
+        EGL_ALPHA_SIZE,           8,
         EGL_RENDERABLE_TYPE,      EGL_OPENGL_ES2_BIT,
         EGL_NONE,
     };

plugins/platforms/hwcomposer/hwcomposer_backend.cpp

@@ -391,7 +391,7 @@ static void initLayer(hwc_layer_1_t *layer, const hwc_rect_t &rect)
 }
 
 HwcomposerWindow::HwcomposerWindow(HwcomposerBackend *backend)
-    : HWComposerNativeWindow(backend->size().width(), backend->size().height(), HAL_PIXEL_FORMAT_RGB_888)
+    : HWComposerNativeWindow(backend->size().width(), backend->size().height(), HAL_PIXEL_FORMAT_RGBA_8888)
     , m_backend(backend)
 {
     setBufferCount(3);