inconsistent return types for initializeOpenGLFunctions()

The return types of initializeOpenGLFunctions() appear to be inconsistent accross various QtGui classes:

void QOpenGLFunctions::initializeOpenGLFunctions(void),
bool QOpenGLFunctions_x_y_*::initializeOpenGLFunctions(void)
void QOpenGLTexturePrivate::initializeOpenGLFunctions(void)

This makes it impossible to share code that checks the return code of initializeOpenGLFunctions() – required for explicitly 'named' profiles – with the default core profile. This is because checking for the return code is not possible for the default QOpenGLFunctions class as compile time errors are triggered ("could not convert void to bool for ...").

The attached patch against 5.2.0-alpha proposes to have consistent bool return type for all occurrences of initializeOpenGLFunctions() and just return 'true' where appropriate.