feat(PanoImageRenderer): Refactoring/tune renderer & add keepUpdate

[jongmoon]

Issue

#89

Details

1. Shorten image load process (<-- It makes better performance)
2. Add keepUpdate interface Add interface for controlling to keep update. #93
3. Refactoring Code

WebGL Command is captured using [WebGL Inspector](http://benvanik.github.io/WebGL-Inspector/

• 
• Remove useless code
  • (like !this.hasRenderingContext() check. because render function should be called after rendering context sucessfull)

[coveralls]

Coverage decreased (-4.6%) to 40.776% when pulling 4d97bc2 on jongmoon:video#89 into 27f24d3 on naver:video#44.

[happyhj]

LGTM!

[coveralls]

Coverage increased (+0.4%) to 45.754% when pulling 6330764 on jongmoon:video#89 into 27f24d3 on naver:video#44.

[coveralls]

Coverage increased (+0.3%) to 45.67% when pulling 85419b8 on jongmoon:video#89 into 27f24d3 on naver:video#44.