Optimize read_vrt window for needs_resample sources #1704

Background

PR #1699 fixed issue #1694 by resampling source pixel data when a SimpleSource's <SrcRect> size differs from its <DstRect> size. The fix reads the full <SrcRect> from the source file before resampling and clipping to the destination window.

Concern

For needs_resample sources, read_vrt now always reads the full SrcRect even when the caller requests a tiny window= (e.g., a 1x1 window). For large SrcRect this can cause a big decode/memory hit.

See xrspatial/geotiff/_vrt.py around the needs_resample branch (currently lines 552-556), where the read window is set to the full SrcRect:

if needs_resample:
    read_r0 = sr.y_off
    read_c0 = sr.x_off
    read_r1 = sr.y_off + sr.y_size
    read_c1 = sr.x_off + sr.x_size

Proposal

Compute the subset of SrcRect that maps to the requested window (via the inverse of the nearest-neighbour mapping used by _resample_nearest) and read only that subset.

Care needed:

• The inverse mapping must include all source pixels that any output pixel within window could sample from. Off-by-one boundary errors here would drop pixels at the window edge.
• Both integer-ratio fast paths and the general non-integer path must agree on the subset.
• The clip math at the end of the needs_resample branch must adjust to work with a partial src_arr whose origin is no longer at (sr.y_off, sr.x_off).

Workaround

Until this is optimized, callers that pass a small window= on a VRT with large-SrcRect SimpleSources will see the full SrcRect read into memory. Reading without window= and slicing the result has the same cost.

Originally raised by Copilot in the review of #1699.