Add support for linking binary assets

[Gadgetoid]

This is an attempt at adding support for linking binary assets, so that map data, graphics, sound, or other useful bits and bobs can be appended to a binary without having to deal with header files.

It's used by invoking blit_asset in an example/project CMakeLists.txt.

blit_asset takes two arguments:

1. The project name
2. The binary file to link into the project

For example, to link the audio rising.raw into doom-fire I might use: blit_asset (doom-fire rising.raw)

There's a much more interesting take on this by @Daft-Freak - Daft-Freak@a373fa4

Which introduces the concept of executing a packer tool (in this case our sprite-builder) to perform build-time conversion of assets. This is really cool since it takes a whole manual step out of the build and avoids the user having to care about how/when their lovely image files are converted.

I think these two approaches should be combined into something that:

• Takes an asset- be it a map, image, data, whatever
• Accepts a "tool name" or "asset type" which governs how that asset will be transformed into binary data
• Executes the tool against the asset
• Converts the binary result into an object file
• And additionally a header file with a convinient pointer at the data for the user to hook into
• Links the result against the project

So a user can just:

#include "my-asset.h"

blit_asset (my-project sprite my-asset)

Or something similar.