Fixed glSampleCoverage signature (vi to vii)

The signature for glSampleCoverage was incorrect (vi); glSampleCoverage takes two arguments (vii).

See: https://www.khronos.org/registry/webgl/specs/1.0.2/#5.14.3
and http://www.khronos.org/opengles/sdk/docs/man/xhtml/glSampleCoverage.xml
Merge pull request #1891 from onnoj/incoming

Fixed glSampleCoverage signature (vi to vii)
do not assume fastcomp emits 'pre' table
finalize function table masks for fastcomp
handle global initializers from fastcomp
shorten test_cubescript for fastcomp now that it passes
fix memset return value in makeSetValues
Tag current state
Remove pc file check
Rename `ensure` to `download`.
Add stub purge and remove commands
Remove name attribute of Version.
Add current code
Add implementation of purge and remove commands.
Use `urllib.urlretrieve` for all fetches, since it supports file:// retrieval already.
Only switch to special handling for file:// URL being a directory.
Thus we no longer need `requests` module.
Set EMSCRIPTEN_ROOT environment variable so executable may be referenced absolutely.
Execute local commands via `sh` so the behavior is similar on all platforms.
Improve handling of package `build` attribute.
Handles non existance and features saner list wrapping.
Introduced wrapper emcmake, which sets the most common configuration flags.
Change test_cmake to use `emcmake`.
Make rename libcache to lib_dir