optimize vec3 parse

[ngokevin]

Description:

Doing a performance profile on allocations using that in-VR perf demo, I found vecParseFloat was the 2nd biggest object allocator, and vecParse was the 6th biggest object allocator. Seems right given how often setAttribute on a vec3 is called, although I would've though skipTypeChecking should've prevented these calls at all.

Changes proposed:

• In vecParseFloat, don't create an Array (Object.keys) and callback. Don't need to call parseFloat on a number.
• In vecParse, move an object allocation down to only when it's needed (string parsing). And save an object allocation by modifying existing object.