Clear is called when an image becomes a render target, and it
looks like Clear causes the memory spikes.
Before this change, when an image is created, shared region on a GL
texture is allocated. After that, the image's region is copied to
non-shared region when the image becomes a rendering target.
After this change, an image is not allocated on a GL texture first,
and allocated only when it is necessary. Then, Clear calls can be
avoided as much as possible.
