Changes:
* Create render library (move util/render.cpp to render/render.cpp)
* Move app::Zoom class to render::Zoom
* Remove doc::Image::merge() member function
* Add gfx::Clip helper class (to clip dst/src rectangles before a blit)
* Move doc::composite_image() to render::composite_image()
* Remove doc::Sprite::render()
* Replace Sprite::getPixel() with render::get_sprite_pixel()
* Remove doc::layer_render() function
* Convert DitheringMethod to a enum class
* Add AppRender to configure a render::Render with the app configuration
* Move checked background preferences as document-specific configuration
* Add doc::Sprite::layer() and palette() member functions
* Add doc::Layer::cel() member function
* Add doc::Palette::entry() member function()
* Add doc::frame_t type
* Move create_palette_from_rgb/convert_pixel_format to render library
* ExportSpriteSheet doesn't need a temporary image now that we can specify
the source rectangle in the render routine
Here we avoid copying and clearing pixels that will not be used
in the whole tool loop process.
Changes:
* Add several member functions in ToolLoop to validate/invalidate regions
of source/destination images so we know what regions are safe to use
by inks and can be shown in the editor
* Add new DocumentObserver::onExposeSpritePixels() member to validate
pixels that will be displayed in the editor
* Add Ink::needs/createSpecialSourceArea() member functions to validate
extra areas for inks like blur or jumble
* Add undoers::ModifiedRegion to save the undo information about the
modified region
* Add ShowHideDrawingCursor class
* Change "blur" tool policy from overlap to accumulate
(This is a real fix for issue #239)
