Use dirty-region tracking for partial framebuffer refreshes (G9, B17)

[Ashex]

Context

The framebuffer layer has skeets_framebuffer_mark_dirty() and skeets_framebuffer_flush() infrastructure in src/platform/framebuffer.h/cpp, but the main rendering loop in src/kobo/main.cpp never uses it. Instead, it does explicit refresh_region() calls for stat lines and full-screen refreshes for everything else.

Feed/thread stat updates already use localized partial GC16 refreshes (render_feed_stats_only / render_thread_stats_only), but view-level redraws (scroll, view switch, image load) still trigger full-screen flashing refreshes.

Sub-tasks

• Transition view rendering to use skeets_framebuffer_mark_dirty() during draw operations
• Call skeets_framebuffer_flush() at end of each render pass instead of explicit refresh calls
• Replace full-screen GC16 refreshes on scroll with partial DU refreshes of the changed region
• Replace full-screen refresh on async image load with partial GC16 refresh of the image region only
• Ensure view-switch transitions still get a full GC16 clear (for clean state between views)

Impact

Reducing unnecessary full-screen e-ink flashing will significantly improve the reading experience — less flickering during scroll, image loads, and interaction toggles.

References

• Dirty region infrastructure: src/platform/framebuffer.h/cpp
• Current refresh calls: src/kobo/main.cpp
• KOReader waveform reference: uses REAGL for partial updates, DU for fast scroll, GC16 for full