Reuse existing Skia binaries if possible

[ghost]

Currently, it seems that every time the skia-bindings crate is recompiled (for example when compiling after upgrading the Rust toolchain), it redownloads Skia, as sudo iptraf reports traffic when cargo reaches skia-bindings (build).

Normally this is not that much of an issue as wherever it's downloading from serves the files quickly enough, but since yesterday it has become excruiciatingly slow and even corruptions occurred.

As per #10 (comment), the build script can be improved by adding a check if the desired binaries are already downloaded.

[pragmatrix]

@JackRedstonia You've mentioned that the download happens every time a compiler flag is changed. Can you provide one example that can be used to prove a working fix? I am asking because I assume that the output directory gets changed for even the smallest environment change with the consequence that downloaded archives must be cached somewhere else.

[ghost]

@pragmatrix Something like this in Cargo.toml of a project that depends on skia-bindings for example:

[profile.release]
lto = "thin"

Commenting/uncommenting this results in a rerun of the build script it seems, which in turn redownloads the binaries, plus some other stuff from codeload.github.com.
I tried cloning the repository and swapping out all the URLs with file://s to local downloads, but it still seems to download something.
(local downloads were obtained through proxies, GitHub always chooses the worst file server to download from for my region, resulting in the build taking forever and then failing on a network error)

[ghost]

Adding new dependencies to a project also rebuilds.