* fixed lint errors in button.go
* fixed lint errors in checkbox.go
* Removed d2ui singleton, fixed nearly all lint errors
- Changed `UI` struct to `UIManager`, removed singleton
- UI element provider functions are now methods of the UI Manager
- Screens now use the UI manager to create UI elements
- game panels in d2player now use the UI Manager to create UI elements
- Only the UI manager knows about "widgets"; calls to `d2ui.AddWidget` in Screen instances have been removed
* changed ui element provider methods from `Create` to `New`
* Add PushEffect, handle effects in renderer
* Set effects instead of blend
* Stop using PushCompositeMode, use PushEffect
* Remove remaining composite mode things
* abstracted palettes, colors, and palette manager
* make asset manager use the palette manager interface
* added BGRA setter/getter, fixed lint errors
* abstraction for animation and animation manager
* Fixes#496 using ebiten.SubImage to Render Section of a Surface.
I had to add matching functions to both animation and sprite to get it to be called
for a sprite object.
* Fixed linter warning on comments for Sprite and Animation.
* Removing what's remaining of the old Sprite re-generation and caching.
