Define Pixmap and PixmapAtlas to abstract image loading/mgmt

[nogginly]

TL; DR
@blacktm, I'd like to propose abstracting image loading (and mgmt) from the renderable Image with the eventual goal of being able to support drawing images (and, later, text) into a Canvas. I'm use the name Pixmap to avoid breaking the current APIs, but as always I'm open to better naming.

Usage

1. Setup a PixmapAtlas once and use it when creating image-like shapes (i.e. Image, Sprite, Tileset) to ensure that an image file is loaded only once. For example:

  ...
  atlas = PixmapAtlas.new
  Sprite.new 'assets/hero.png', atlas: atlas
  ...

2. Canvas can draw Pixmaps, which can be created directly, or even better, can be created using a PixmapAtlas to ensure the same image doesn't get loaded more than once. For example:

  ...
  atlas = PixmapAtlas.new
  hero = atlas.load_and_keep_image('assets/hero.png') as :hero
  canvas.draw_pixmap atlas[:hero], x: 10, y: 100 
  ...  

Changes/commits

• New
  • Define a Pixmap which can be used to load an image file and convert to Texture when needed
    • ✅ Add tests
  • Define a PixmapAtlas which can be used to manage loading/caching Pixmap by name.
    • ✅ Add tests
  • Add Canvas#draw_pixmap method to draw a Pixmap with resizing
    • ✅ Support drawing cropped pixmap into Canvas so portions of a pixmap can be drawn. (See coin in screenshot below)
• Changes
  • Image supports a load_image_as_texture class method which, given a file path and optional PixmapAtlas, will use Pixmap to load the image and track it in the atlas by file path.
    • ✅ Expand test specs to verify atlas
  • Sprite and Tileset modified to use above Image loader method and support optional atlas in constructor
  • Converting image to GL texture modified to support GL_BGR for non-OpenGL ES, leaving in the pixel manipulation otherwise.
• Removals
  • Removed the native Image.ext_load_image class method
    • Native image loading is done by Pixmap.ext_load_pixmap

I realise this is a "design" change, so I am completely open to changes that better align with the goals of Ruby2D.