add simple example to render text using `fontdue`

[Nevsden]

Added text rendering in the most simple way using fontdue to create a text bitmap on request. Also added a simple example to render text on a sprite. Related to #152

• Added a text module under crate::renderer. Not sure, if this is the correct level.
• Added a Font class, which reads in a ttf font using fontdue and can rasterize characters.
• Added a TextEffect holding information on the text and text size to be rendered.

Issues so far with this solution:

• Text is created by accessing Font::rasterize, which could be expensive. An interim struct to hold all possible characters in a texture atlas is probably the more efficient way.
• Not sure, on which level in the crate to implement the text functionality.
• Don't yet know, how to render text on a transparent background, such that the text is the only colorized object in its own visual scope.

[Nevsden]

Changed the struct TextEffect to hold a string instead of a single char. Further changed function rasterize to turn string instead of a char into a CpuTexture. Computation of the correct permutation from the bitmap for each char in the string to the unified TextureData::RgbaU8 brought a little confusion but I solved it.

• Further investigation needed on how to render the text with a CpuTexture without using Sprites.
• Also need to look how to stretch text appropriately on the three-d::Texture2D level, once the CpuTexture has been consumed by the ColorMaterial::new_transparent function.

Also, todos from @asny still ongoing.

[guija]

I'm interested in this change, what is the status of this? Is someone working on it? I could support if necessary.

[asny]

I'm interested in this change, what is the status of this? Is someone working on it? I could support if necessary.

I think this is mostly replaced by https://github.com/asny/three-d/issues/376, so I'm closing this.