point3d: Use SIMD ops for scale()
point3d: Use SIMD ops for interpolate()

Given that we're operating on the equivalent of a vec3, it's going to be
more efficient to just use a SIMD4f and then read back.
Begin the slow process of documenting Graphene

Each type should have its own section, and each public symbol should
have a propert annotation.
tests/matrix: Use get_value()
tests/point: Add more code coverage
Add support for code coverage of the test suite

We use lcov and GCC to generate code coverage reports after running the
test suite.
tests/point: Add full code coverage