fix shared object build with -fvisibility=hidden

set WEBP_EXTERN to visibility=default
+ explicitly mark VP8GetCPUInfo as it's referenced within the examples

Change-Id: Ie3d2b15088e888f0b55203b205993eba75899d99
Simplify and speedup Backward refs computation.

Updated VP8LGetBackwardReferences and HashChainFindCopy method with following:
- Remove the recursive CostModelBuild.
- Reuse the lz77 backward refs in CostModelBuild, instead of evaluating it
  again (as it was done for recursion_level=0).
- Consolidated the Match-length logic inside FindMatchLength method.
- Removed the logic for altering best_length/val based on the 2D distance.
  The additional 162 value (+= 9 * 9 + 9 * 9 - y * y - x * x) can't change the
  best_val eval computation to choose a different curr_length, as best_val was
  set to 'curr_length << 16'.

  Following is the impact on the compression speed/density at default & max
  quality, overall this speeds up compression by 5-15% (q=100 -> 75) with a tad
  drop (0.02-0.03%) in compression density for the non-palette images.

                  Before                After
                bpp/Rate(MP/s)        bpp/Rate(MP/s)
q=75 (def)
All 1000        2.4492/1.049 MP/s     2.4498/1.230 MP/s
Palette         0.2719/5.060 MP/s     0.2719/6.110 MP/s
non-Palette     3.7597/0.732 MP/s     3.7607/0.840 MP/s

q=100
All 1000        2.4134/0.125 MP/s     2.4142/0.131 MP/s
Palette         0.2692/2.585 MP/s     0.2692/2.885 MP/s
non-Palette     3.7040/0.079 MP/s     3.7053/0.083 MP/s

Change-Id: I27a5eff3356d876c3e949fd32262244b25678b7a