[Impeller] Fix Windows build with MSVC 14.52: std::modf is [[nodiscard]] - #193413

The MSVC 14.52 STL declares std::modf as [[nodiscard]] (14.51 does not). RoundToHalf in line_geometry.cc called std::modf only for its integral part output parameter and discarded the return value, which breaks the Windows engine build because warnings are treated as errors:

../../flutter/impeller/entity/geometry/line_geometry.cc(86,3): error: ignoring return value of function declared with 'nodiscard' attribute [-Werror,-Wunused-result]
   86 |   std::modf(x, &whole);
      |   ^~~~~~~~~~~~~~~~~~~~

Use std::trunc instead, which returns the integral part directly and yields the same value as std::modf's integral part for all inputs, including infinities and NaN.

Related to #96138.

For transparency: This PR was partially created with help of Claude.

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