perf: utilities_get_rgba_for_xy() allocates a GdkPixbuf per pixel read

[killerdevildog]

Problem

utilities_get_rgba_for_xy() in src/utilities/utilities_paths.py calls Gdk.pixbuf_get_from_surface(surface, x, y, 1, 1) to read a single pixel. This allocates a full GdkPixbuf GObject on the heap for every call.

utilities_get_magic_path() (used by the paint bucket / magic wand) calls this function up to 50,000 times per operation, making GObject allocation the dominant cost.

Proposed Fix

Replace the GdkPixbuf allocation with a direct read from the Cairo surface pixel buffer via surface.get_data(), handling the BGRA→RGBA conversion and alpha un-premultiplication in Python.

Cache the buffer once in utilities_get_magic_path() instead of re-acquiring it per pixel.

Measured Impact

Benchmarked on Python 3.12.3 / Cairo 1.18.0 / GTK 3 / Linux:

Scenario	Before	After	Speedup
Single pixel read (median)	3.5 μs	0.78 μs	4.5×
10,000 consecutive reads	36.4 ms	7.1 ms	5.1×
Paint bucket worst case (50K reads, projected)	~180 ms	~36 ms	~5×

Also fixes an off-by-one in the bounds check (x > width → x >= width).

Affected Tools

• Paint bucket / magic wand (heaviest user — 50K calls per fill)
• Color picker (per mouse-move)
• Eraser color comparison