quaternion: Add to_angles() conversion

We have initializers from various formats (Euler angles, angle/axis,
matrix), but we are missing a conversion function from quaternion to
Euler angles.
quaternion: Use deg-to-rad macro
quaternion: Check out arguments

They are not documented as being optional, but we can relax the
requirement in the future, if needed.
vec: Add negate() operator
docs: Fix typo
vec: Add equality check function

We want to check if two vectors are the same, or have the same values.
Add a plane type

Planes are useful in fully 3D canvases, as they can be used for things
like frustum-based culling.

The graphene_plane_t data structure represents a plane via a normal
vector pointing towards the origin of the plane; and a constant value
representing the distance from the origin along the normal vector.
Fix typo in the comment
plane: Add more initializers
build: Bump up the version

We added new API, and we already branched, so we need to bump up the
configure.ac version.