The animated pattern math (background sweep, moving 24px bar, frame-
counter block) was copy-pasted across all five clear-based backends
(dx9/dx10/dx11/dx12/gl). The backends MUST render an identical image --
the capture test decodes the same frame-counter block from each -- so a
drift between copies would be a silent test hazard.
Factor it into one frame_pattern() helper (plus kBarWidth) in
render_backend.hpp; each backend now consumes the computed bg/bar_x/code
values with its own API's clear/fill. mock_game_test still decodes every
backend correctly.
BlackMark
9a736cf477