Allow the player to place and break voxels

[patowen]

The main purpose of this PR is simple: allow the player to break blocks by left-clicking on them and allow them to place blocks by right-clicking on other blocks.

To enable this feature, there are several changes/additions (most important ones listed here):

• Added a new ray tracing function to figure out what block is being clicked
• Refactored collision checking code to allow some code-reuse by ray-tracing code
• Removed use of chunk margins in collision-checking code and tried to adopt a consistent coordinate system of [u8; 3] without margins. Rendering code is unaffected by this change (margins were not removed entirely).
• Added some methods to DualGraph to simplify common operations related to placing/breaking blocks
• Added surface invalidation logic so that updating blocks reruns surface extraction
• Partially implemented saving of voxel data (Loading logic doesn't yet exist, so this is not yet testable or useful).

There is also a minor cosmetic change, reducing ambient occlusion to make textures at the ends of 1x1 tunnels visible instead of being pitch black.

Fixes #292

[patowen]

This PR is probably still worth reviewing, but I'll update it to a draft because it doesn't properly handle the case of Player B modifying a chunk that Player A has not generated and then Player A visiting that chunk later. I plan to address this later, and I'll check off the below task once I've done so:

• TODO: Fix server-reported block updates in ungenerated chunks

Another issue is that for new clients that receive full chunk data from the server, there is no logic to remove the margins of chunks adjacent to any edited blocks, so this can result in visual bugs. Ideally, the solution to this does not involve the server in any way, since the server shouldn't need to care about margins. The solution to this doesn't need to be too efficient, as margin handling is still a work in progress to begin with.

• TODO: Remove margins of all chunks adjacent to edited chunks, even those generated later.

[patowen]

I decided to remove the code that saves voxel edits in favor of putting that code in the PR that also loads voxel edits, since that way, the code will actually be tested.

[Ralith]

Just a few cosmetic issues; otherwise LGTM!