renderer: re-use the same opengl instance for all background rendering

Previously the OpenGL instance was tied to each render/cache task,
creating and destroying it each time one started and stopped. This was
completely unnecessary since the instance holds no state and can be
shared by all of the render tasks without having to expensively start
a new one.

app/render/backend/opengl/openglworker.cpp

@@ -22,15 +22,14 @@
 
 OLIVE_NAMESPACE_ENTER
 
-OpenGLWorker::OpenGLWorker(RenderBackend *parent, OpenGLProxy* proxy) :
-  RenderWorker(parent),
-  proxy_(proxy)
+OpenGLWorker::OpenGLWorker(RenderBackend *parent) :
+  RenderWorker(parent)
 {
 }
 
 void OpenGLWorker::TextureToFrame(const QVariant &texture, FramePtr frame, const QMatrix4x4& mat) const
 {
-  QMetaObject::invokeMethod(proxy_,
+  QMetaObject::invokeMethod(OpenGLProxy::instance(),
                             "TextureToBuffer",
                             Qt::BlockingQueuedConnection,
                             Q_ARG(const QVariant&, texture),
@@ -42,7 +41,7 @@ QVariant OpenGLWorker::FootageFrameToTexture(StreamPtr stream, FramePtr frame) c
 {
   QVariant value;
 
-  QMetaObject::invokeMethod(proxy_,
+  QMetaObject::invokeMethod(OpenGLProxy::instance(),
                             "FrameToValue",
                             Qt::BlockingQueuedConnection,
                             Q_RETURN_ARG(QVariant, value),
@@ -58,7 +57,7 @@ QVariant OpenGLWorker::CachedFrameToTexture(FramePtr frame) const
 {
   QVariant value;
 
-  QMetaObject::invokeMethod(proxy_,
+  QMetaObject::invokeMethod(OpenGLProxy::instance(),
                             "PreCachedFrameToValue",
                             Qt::BlockingQueuedConnection,
                             Q_RETURN_ARG(QVariant, value),
@@ -71,7 +70,7 @@ QVariant OpenGLWorker::ProcessShader(const Node *node, const TimeRange &range, c
 {
   QVariant value;
 
-  QMetaObject::invokeMethod(proxy_,
+  QMetaObject::invokeMethod(OpenGLProxy::instance(),
                             "RunNodeAccelerated",
                             Qt::BlockingQueuedConnection,
                             Q_RETURN_ARG(QVariant, value),