vg ector: Call a pair of ector begin/end for ector buffer drawing.

vg ector: Call a pair of ector begin/end for ector buffer drawing.

While we render vector drawing on ector,it uses ector buffers for subsequent compositions.
In this sequence, it switches ector buffer with a composition target,
while resuing the global ector context.

Previously, it trickly switched ector buffer, it broke the ector begin/end pair.
Now, it needs to recover global ector buffer once the composition finished.
Thus ector buffer need to return the current pixels information for this.

Change-Id: I8029ad548660fe614923857af9520e67bd9344a1