Issue 104. Image surface caching.

Incorporated Henry's comments/suggestions on image surface caching. 
Observed good difference in fps before and after change for EGL. Image backend results are same as before.

EGL (PC: Before)
cairo-image          100    PASS  100    55
cairo-image_scale    100    PASS  100    27
cairo-image_rotate   100    PASS  100    22
cairo-clip           100    PASS  100   261

EGL (PC: After)
cairo-image          100    PASS  100  1029
cairo-image_scale    100    PASS  100   811
cairo-image_rotate   100    PASS  100   782
cairo-clip           100    PASS  100   818