Fix QOpenGLGlyphTexture object leak

Call clear() in the destructor of QOpenGLTextureGlyphCache class to
prevent QOpenGLGlyphTexture object leak.

Change-Id: I290b09b0786d30603391e6855a21e9232b112739
Reviewed-by: Gunnar Sletta <gunnar.sletta@jollamobile.com>