Return an error instead of panicking on invalid transform-origin

[ChrisJr404]

TransformOrigin::from_str can panic on a number of short, otherwise-innocuous inputs. For example:

use std::str::FromStr;
use svgtypes::TransformOrigin;

let _ = TransformOrigin::from_str(" ");  // panics
let _ = TransformOrigin::from_str("\t"); // panics
let _ = TransformOrigin::from_str("-");  // panics
let _ = TransformOrigin::from_str("+");  // panics

Each of these hits the unreachable!() arm:

thread 'main' panicked at src/transform_origin.rs:184:18:
internal error: entered unreachable code

Why it happens

At the top of from_str we bail out early only when the input is completely empty. For a non-empty input the first component is parsed with parse_part, which tries parse_directional_position and then parse_length. parse_length calls skip_spaces() (and consumes a leading sign) before it fails, so on an input like " " or "-" the parse returns Err/None but the stream cursor has already advanced to the end.

Because the stream is now at its end, both of the if !stream.at_end() blocks that would parse the second and third components are skipped, leaving first_arg == None and second_arg == third_arg == None. The final match only handles the (Some, None, None) and (Some, Some, _) shapes, so this falls through to _ => unreachable!() and panics.

Since from_str runs on untrusted attribute values (e.g. a transform-origin presentation attribute), a caller that does not wrap the parse in catch_unwind can be taken down by a one-byte input.

Fix

Return the already-defined TransformOriginError::InvalidParameters for that fall-through case instead of panicking. Valid inputs are unaffected — they always produce a Some first component and hit the existing arms.

Added regression tests for " ", " ", "\t", "-", and "+". The full test suite passes and cargo fmt/cargo clippy --lib --tests are clean.