Numerical precision issue in rotate_vector

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Hi,

I ran into a numerical precision issue when using the rotate_vector function with 32bit floats as input data. In this case, the lat/lon increments used to determine the relative rotation between map projection and the lat/lon grid is chosen too small. The result was a cristalline structure in the plotted wind barbs which patches of barbs pointing in the exact same direction.

https://github.com/matplotlib/basemap/blob/decfa95124dab76499734145d03b002b9db27477/lib/mpl_toolkits/basemap/__init__.py#L3118

Converting the input to 64bit floats solves the issue for me, as did an increase of the increment to 1.0e-4.

Best regards,
Clemens

从 lib/mpl_toolkits/basemap/__init__.py 中 issue 所引用的 rotate_vector 实现附近开始。使用 32 位浮点输入复现风羽伪影，将其与 64 位输入和提到的更大增量进行比较，并验证在不改变预期旋转行为的情况下，晶体状图案已被消除。

由索引模型根据 Issue 内容生成。