Helpers overhaul

[bzm3r]

Per discussions, an overhaul of helpers that also with the following issues:

• Feature Request: API to translate map coords to screen coords #30

• Layout of hex tiles is inaccurate, with error accumulating #58

• Make neighbour helpers public #278

It provides the ground work for dealing with (but does not yet generate mouse events):

• Feature request: mouse-to-tile events #40

What remains before the PR can move out of draft stage:

• docs need to be updated/cleaned up (there are a lot of docs around that no longer belong where they are, or need to be simplified)

• update hex shader code to use new tile position to world position code for hexagons

• create an example that uses the new code, and confirms that world position -> tile position works correctly

An extension to this PR would create mouse events as requested in #40

[bzm3r]

Brief overview:

• helpers have been re-organized so that they are not in a single file

• for hex grids: added support for conversion between axial coordinate system (the coordinate system used by Row and Column hex coord system) and offset coordinate systems (RowEven, RowOdd, ColumnEven, ColumnOdd)

• for hex grids: added support for conversion between axial coodinate system (the coordinate system used by Row and Column hex coord system) and cube coordinate systems

• for hex grids: added support for conversion between axial/cube/offset coordinates and TilePos

• for hex grids: note that all these conversions between coordinate systems are cheap and require only a couple of arithmetic operations at most (maybe I should be adding the #[inline] hint everywhere, but I'll make a future issue for that)

• for hex grids: supporting cube positions allows us to convert a Vec2 world space position into a TilePos; suppose you are using RowOdd coordinate system, and you have a world_pos, then: world_pos is mapped to a FractionalAxialPos, which is then rounded (requires conversion to FractionalCubePos at this stage) into an AxialPos (this rounding is actually the heavyweight bit), which is then converted into a RowOddPos, which is converted into a result TilePos

• for hex grids: floor/ceil are no longer used to lay out the tiles (I think this was responsible for the layout error accumulation?)

• for isometric grids: simplified how the math around tile layout by capturing the square -> isometric projection as a matrix, this allows us to apply a matrix inverse to easily calculate which tile a Vec2 world position belongs to (essentially, the squishes the isometric grid back to a square grid) ---- this process is also quite similar to what is happening for hex grids, so there is some unification of code between hex grid/iso grid shaders!

• for squares/isometric grids: tile center is now at (0, 0)

• a lot of the shaders are now quite similar, because ultimately, the math behind them has deep similarities

• for neighbor helpers: made lots of private functions public because Make neighbour helpers public #278

[StarArawn]

If our tiles are centered now do we need this? Or is that only the case for hex/iso?

[bzm3r]

I am not sure, but I think so? This is for centering the whole tilemap, right? Currently, our tiles are "centered" in the sense that the center of a tile with TilePos(0, 0) lies at [0, 0] in world space.