Functions to rely less on explicit UIGraphicsGetCurrentContext() calls.
Significantly improves AZTableViewCell speed.

 - Shadow users should manipulate the tableViewBackgroundColor property.
 - Refactors drawing code, fixes numerous bugs
 - Cell backgrounds are often opaque, and no longer overlap by design.
 - Drawing code are now (internal) functions for future use.
 - Bugs fixed which caused needless image creation and cache filling
Fixes table view demos.