Paint: Flood Fill improvement

[Vutshi]

Add FrontFill algorithm (on mouse click)
Add benchmark_mode flag
Fix canvas bounds check in R_LineFloodFill (keep it on 'f' button)

[Vutshi]

The front_fill function performs a flood fill on a 2D canvas, starting from an initial point (x0, y0), replacing all connected pixels of a target color with a new_color. It uses a scanline approach, processing the image row by row by tracking horizontal segments of pixels.

S.V. Burtsev, Ye.P. Kuzmin,
An efficient flood-filling algorithm,
Computers & Graphics, Volume 17, 5, 549-561 (1993)

Key Components:

1. Data Structures:

• Segment: Represents a horizontal run of pixels with left (xl) and right (xr) bounds.
• SegList: Groups segments on a specific row (y) with a direction (dir: +1 for down, -1 for up) and a list of segments (s).

2. Helper Functions:

• expand and expand_right: Finds the maximal horizontal run of target pixels around a point (x, y).
• fill: Paints all pixels in a SegList’s segments with new_color.
• link: Generates a new SegList for the next row (E.y + E.dir), finding segments vertically adjacent to those in E.s.
• diff: Computes segments in Ep not overlapping with Ed’s segments (expanded by 1 pixel on both sides), identifying new areas to explore in the opposite direction.
• merge: Combines two SegLists on the same row and direction into one, efficiently merging sorted, non-overlapping segment lists.

3. Main Algorithm (front_fill):

• Initialization: Starts by expanding around (x0, y0) to form an initial segment, creating E_plus (downward) and E_minus (upward) SegLists, and filling the initial segment.

• Stacks: Uses two stacks, S_plus and S_minus, assigned to active (Sa) and passive (Sp) roles, swapping them to balance workload.

• Loop:
  - Pops a SegList (E) from Sa.
  - Links to the next row’s segments (Ep), fills them, and computes the difference (Em) as new frontiers in the opposite direction.
  - Merges Ep into Sa if on the same row as the top of Sa, otherwise pushes it.
  - Pushes Em to Sp.
  - Swaps Sa and Sp if Sp has more segments, balancing processing.

• Return: The maximum capacity of Sa encountered, this is a temporary diagnostic

[ghaerr]

Nice catch on CANVAS_HEIGHT in R_LineFloodFill! I'm surprised that bug made it this long :)

Pretty amazing work and code on the new fill algorithm - wow! I'm traveling and haven't had time to review it, but look forward to understanding it. I will also be very surprised if that code makes it through the C86 compiler, but that's ok if it does not.

Thank you!