cabana: optimize sparkline rendering by reducing points in horizontal segments

[deanlee]

This PR improves sparkline performance by removing redundant points in horizontal segments (i.e. where Y values remain constant), while preserving visual accuracy.

For example, rendering a 100 Hz constant signal over 30s previously drew ~3000 lines; with this PR, it's reduced to just one.

Benefits

• Reduces point count by 50–90% for signals with flat segments (common in CAN data)
• Improves rendering speed by reducing drawPolyline overhead
• Enables antialiasing more often (under 500-point threshold)
• Cuts Cabana CPU usage by ~10%