evas vg: revise buffer caching mechanism from ector version.

evas vg: revise buffer caching mechanism from ector version.

This helps to save memory when process uses the same resource
among the objects.

In case of ui, many ui instances uses the same visual resources,
So this mechanism is very effective.

Additionally, this keeps the designated buffer for animation frames.

Previously, we didn't reuse the buffer so every frame buffer is
generated and destroyed, it's an inefficient task...

Change-Id: Ida6c59d972ccc130cb0fa990fce475ce5a2dca64