Implement SpriteSystem.

[Possseidon]

Implement SpriteSystem.

• TODO: Add support for a scissor test of sorts.
  • Think Scrollbox in a GUI or even just a textbox with left/right scrolling.
  • Possibly have a smooth transition on the border.
• TODO: Properties with animation support.
  • Scripting is okay for setting values directly.
  • ... but animations should be smooth and efficient.
  • Create some new Property class in dang-utils or similar.
• TODO: Elaborate on this in general.

Shader Layout

• Positioned freely on XY.
• Z for ordering.
  • Any translucent sprites must be sorted and rendered back to front.
  • Opaque/transparent sprites can theoretically be rendered AFTER in arbitrary order.
  • However, then the order for same Z is not consistent.
  • Instead sort all and keep using the add-order by using stable sorting.
  • Maybe improve on this in some way in the future...
• Tinting.
• Texture.
  • Two textures for fading.
  • Texture opacity, which can be 0 to disable texture lookup entirely.
  • Texture opacity is NOT applied to alpha, as this would not make much sense.

Shader Attributes

From the list above, these attributes are going to be required for a sprite shader:

• vec3 pos;
• vec4 tint;
• vec2 texcoord1;
• vec2 texcoord2;
• float texture_fade;
• float texture_opacity;