[platforms/virtual] Add support for EGL_MESA_platform_surfaceless

Summary:
Thus we don't require vgem any more which fails with latest mesa.
Kepping the vgem variant as fallback for older systems. See T10245

Test Plan:
SceneOpenGL Test passes without init vgem, not tested with
latest mesa

Reviewers: #kwin

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D17980

TESTING.md

@@ -11,7 +11,8 @@ The following additional software needs to be installed for running the test sui
 * breeze window decoration
 
 # Preparing a run of the test suite
-Please load the kernel module "vgem". This is required to provide a virtual OpenGL device.
+In case your system does not support the EGL extension EGL_MESA_platform_surfaceless,
+please load the kernel module "vgem". This is required to provide a virtual OpenGL device.
 
     sudo modprobe vgem
 

plugins/platforms/virtual/egl_gbm_backend.cpp

@@ -99,6 +99,13 @@ bool EglGbmBackend::initializeEgl()
     // Use eglGetPlatformDisplayEXT() to get the display pointer
     // if the implementation supports it.
     if (display == EGL_NO_DISPLAY) {
+        // first try surfaceless
+        if (hasClientExtension(QByteArrayLiteral("EGL_MESA_platform_surfaceless"))) {
+            display = eglGetPlatformDisplayEXT(EGL_PLATFORM_SURFACELESS_MESA, EGL_DEFAULT_DISPLAY, nullptr);
+        }
+    }
+    if (display == EGL_NO_DISPLAY) {
+        qCDebug(KWIN_VIRTUAL) << "Failed to create surfaceless platform, trying with vgem device";
         const bool hasMesaGBM = hasClientExtension(QByteArrayLiteral("EGL_MESA_platform_gbm"));
         const bool hasKHRGBM = hasClientExtension(QByteArrayLiteral("EGL_KHR_platform_gbm"));
         const GLenum platform = hasMesaGBM ? EGL_PLATFORM_GBM_MESA : EGL_PLATFORM_GBM_KHR;