Add an example to exercise tessellation shader support

Also extended Renderer and friends to support setting the number
of vertices per patch with glPathParameteri(GL_PATCH_VERTICES, n).
To support this QMeshData now also stores the vertices per patch.

This demonstrates how to tessellate using quad tessellation mode.

Follow-up commits will add objects to show triangle and isoline
modes.

Change-Id: I68afb31815db430d0b4d8d9cc525efcfb9a216fc
Reviewed-by: Paul Lemire <paul.lemire@kdab.com>

1 files changed, 8 insertions, 0 deletions

diff --git a/src/render/backend/qgraphicshelpergl2.cpp b/src/render/backend/qgraphicshelpergl2.cpp
index b7b30a52c..68be92cf2 100644
--- a/src/render/backend/qgraphicshelpergl2.cpp
+++ b/src/render/backend/qgraphicshelpergl2.cpp
@@ -120,6 +120,12 @@ void QGraphicsHelperGL2::drawArrays(GLenum primitiveType,
                           count);
 }
 
+void QGraphicsHelperGL2::setVerticesPerPatch(GLint verticesPerPatch)
+{
+    Q_UNUSED(verticesPerPatch);
+    qWarning() << "Tessellation not supported with OpenGL 2";
+}
+
 void QGraphicsHelperGL2::useProgram(GLuint programId)
 {
     m_funcs->glUseProgram(programId);
@@ -270,6 +276,8 @@ bool QGraphicsHelperGL2::supportsFeature(QGraphicsHelperInterface::Feature featu
     switch (feature) {
     case MRT:
         return (m_fboFuncs != Q_NULLPTR);
+    case Tessellation:
+        return false;
     default:
         return false;
     }