ProgressBar could use less memory, operate more efficiently

The ProgressBar is a 1×1 tilegrid configured to display a W×H bitmap. Instead, it could be a W×1 tile grid, displaying 1×H slices from a 3×H bitmap.

Three tall elements are needed: One which is entirely the outline color; one which is the outline color at the top and bottom and transparent/black in the middle, and one that is the outline color at the top and bottom and the bar color in the middle.

This should reduce memory consumption (from W×H to W+3×H times some constant) and speed updates, since a progress change P pixels wide would only have to update P tilegrid entries instead of P×H bitmap entries. (however, the amount of data actually sent over SPI/I2C to the display would not change)

Start at the ProgressBar implementation and trace how its tilegrid and bitmap are created and updated. Compare the current memory use and progress-update path with the proposed W×1 tile grid and 1×H slices. Done means the display remains visually equivalent while using the proposed lower-memory representation and updating only the affected width.

Written by the indexing model from the issue text.