Modify base::convert_to() undefined implementation

GCC shows divide-by-zero warnings, so it is better in this way.
Fix ui::Manager::pumpQueue() error when compiling in debug mode
Merge branch 'dev' of git@github.com:dacap/aseprite.git into dev
Encapsulate Image implementation (private members, accessors, iterators, etc.)

- Rename _rgba/_graya to raster::rgba()/graya()
- Add raster::color_t type (alias for uint32_t)
- Rename raster::GfxObj to Object. And GfxObj::getType() to Object::type()
- Move conversion from raster::Image/Palette to Allegro BITMAP/RGB
  to raster/conversion_alleg.h file
- Add raster/color_scales.h
- Rename image_* functions to raster/primitives.h
- Reimplement ink processing with templates instead of macros
Merge branch 'dev' of git@github.com:dacap/aseprite.git into dev