Compress shader source code

[jfirebaugh]

Reduce binary size by compressing shader source, and decompressing at startup. I split it into two commits to see the difference between compressing each program individually versus compressing the entire corpus at once. The latter is significantly better.

[mb12]

@jfirebaugh On most platforms the final application package is already compressed. The compression is run on all native libraries (incl mapbox-gl.so) and any resources. Any static data added by shader sources will be compressed with other static data, resulting in lower .apk size.

Can you check the sample application .apk size with and without these? There should be little to no difference. Infact the .apk size might be slightly higher with these changes since the compression algorithm would find fewer redundancies with pre-compressed shaders.

[jfirebaugh]

mapbox-gl.so is uncompressed on disk at install time though, so this is still worthwhile.

[mollymerp]

This looks good to me.

I'm not very familiar w the tradeoffs involved w compression/decompression, but are there any drawbacks to decompressing the entire shader corpus for each individual shader with the call to source()? Would decompressing once and then individual shaders referencing the same saved decompressed corpus be possible?

[jfirebaugh]

Would decompressing once and then individual shaders referencing the same saved decompressed corpus be possible?

This is what's currently implemented -- the corpus contains a null byte between each program, so that each shader can be a const char * pointing to the start location within the corpus.

We could potentially implement a strategy where only the programs that are used are decompressed, but this would require additional complexity, like an archive format that allows file-by-file decompression. Probably not worth it.

[friedbunny]

On iOS, a size report for an empty app with our dynamic framework shows that this change has virtually no effect on the compressed app size (i.e., what a user will first download) and a small effect on the uncompressed app size (what is stored on a user’s device).

	Compressed App	Uncompressed App
Uncompressed Shaders	1798.63 kB	5095.10 kB
Compressed Shaders	1798.21 kB	5028.74 kB
	-0.43 kB	-66.34 kB

This is in line with expectations, I’d say.