src/svgtiny_gradient.c: be more careful with float -> int assignment

src/svgtiny_gradient.c: be more careful with float -> int assignment

Assigning the value of a float to an unsigned int is undefined
behavior unless the value is guaranteed to fit. We run afoul of this
in compute_grad_points(), where the number of steps is computed using
a floating point calculation.

On a RISC-V / musl system, the end result is that we wind up with
steps = the maximum value of an unsigned int, when really this should
be an error case resulting in steps = 1. The test suite catches this.