rect: Fix the rect initializer

We use the 'init_from' convention for initializers; the init_from_rect()
init function was correctly declared in the header, but incorrectly
defined in the source file.
tests: Add initial graphene_rect_t test suite
rect: Validate arguments for equals()

We need to check for NULL before dereferencing pointers.
tests/rect.c: Complete coverage for the rect tests