Optimize glyph row clearing and copying routines.
* dispextern.h (struct glyph_row): Change layout of struct glyph_row to help copy_row_except_pointers. Adjust comment. * dispnew.c (null_row): Remove. (clear_glyph_row): Use offsetof and memset to find and clear just the members that need clearing. Adjust comment. (copy_row_except_pointers): Likewise for copying.
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