This is a part of the breaking changes proposed in PR #6865, solving the issue regarding the order of affine transformations described in #2565. This PR also fixes the affected code within Godot codebase. Includes improvements to documentation too.
Another change is, Matrix3::get_scale() will now return negative scaling when the determinant of the matrix is negative. The rationale behind this is simple: when performing a polar decomposition on a basis matrix M = R.S, we have to ensure that the determinant of R is +1, such that it is a proper rotation matrix (with no reflections) which can be represented by Euler angles or a quaternion.
Also replaced the few instances of float with real_t in Matrix3 and Transform.
Furthermore, this PR fixes an issue introduced due to the API breakage in #6865. Namely Matrix3::get_euler() now only works with proper rotation matrices. As a result, the code that wants to get the rotation portion of a transform needs to use Matrix3::get_rotation() introduced in this commit, which complements Matrix3::get_scaled(), providing both parts of the polar decomposition.
Finally, it is now possible to construct a rotation matrix from Euler angles using the new constructor Matrix3::Matrix3(const Vector3 &p_euler).
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