support for glPrimitiveRestartIndex in renderer

glPrimitiveRestartIndex allows more efficient rendering of Strip-based elements, e.g., LineStrip, TriangleStrip, so that you can include breaks in the lines without requiring multiple elements. Here is my quick hack to get this working:

diff --git a/src/render/backend/qgraphicshelpergl3.cpp b/src/render/backend/qgraphicshelpergl3.cpp
index 3fce362..c263103 100644
— a/src/render/backend/qgraphicshelpergl3.cpp
+++ b/src/render/backend/qgraphicshelpergl3.cpp
@@ -67,6 +67,11 @@ void QGraphicsHelperGL3::initializeHelper(QOpenGLContext *context,
m_tessFuncs.reset(new QOpenGLExtension_ARB_tessellation_shader);
m_tessFuncs->initializeOpenGLFunctions();
}
+
+ // Randy's hack to support primitive reset index:
+ m_funcs->glPrimitiveRestartIndex(0xFFFF);
+ m_funcs->glEnable(GL_PRIMITIVE_RESTART);
+
}