[qpa] Don't specify EGL_SURFACE_TYPE when choosing EGLConfig

Summary:
Since QPA only performs off-screen rendering, it does not require an EGLConfig
that supports any particular surface type.  Currently it requests a config
supporting window surfaces, however the EGLDeviceEXT platform, which is the
basis of the EGLStream back-end used with the proprietary NVIDIA driver, does
not support such surfaces resulting in eglChooseConfig not returning any valid
configs when that back-end is in use. Since QPA doesn't actually create any
window surfaces, it should be fine to remove this restriction to allow it to
initialize properly with the EGLStream back-end.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin, davidedmundson

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D20455

plugins/qpa/abstractplatformcontext.cpp

@@ -43,7 +43,7 @@ static EGLConfig configFromGLFormat(EGLDisplay dpy, const QSurfaceFormat &format
 #define SIZE( __buffer__ ) format.__buffer__##BufferSize() > 0 ? format.__buffer__##BufferSize() : 0
     // not setting samples as QtQuick doesn't need it
     const EGLint config_attribs[] = {
-        EGL_SURFACE_TYPE,         EGL_WINDOW_BIT,
+        EGL_SURFACE_TYPE,         0,
         EGL_RED_SIZE,             SIZE(red),
         EGL_GREEN_SIZE,           SIZE(green),
         EGL_BLUE_SIZE,            SIZE(blue),