Fixes for native client/PPAPI Linux

1) A few places assumed REGAL_SYS_PPAPI meant Windows/MSVC when in fact
Pepper is a valid option on Mac and Linux as well. I switch to testing _WIN32 instead, though perhaps that isn't good enough.

Note also that so far I've only been compiling on a Linux environment, but this was a small enough change that I believe I didn't break the Windows builds, but it would be a good idea to check.

2) Initialize RegalContextInfo::maxVaryings to zero before calling GetIntegerv. The PepperGL implementation of GetIntegerv asserts that the output memory is initialized to zero or negative one, though that seems like an odd requirement for a function that is about getting information.