first try at mipmap support for pix_texture
glew-check whether mipmap is supported

according to `glewinfo`, the glGenerateMipmap() is part of the
 GL_ARB_framebuffer_object extension.
 so we use that to set the m_canMipmap in isRunnable(), and && with the
 other conditions (!isRectangle) to determine whether we really want to
 use mipmapping.
 since this knowledge is no longer part of the interal object state,
 we have to pass the "doMipmap" explicitely to setTexFilters()