fix: Consistent wp usage in raster RGB extraction (fixes #1349)

[krystophny]

fix: Consistent wp usage in raster RGB extraction (fixes #1349)

Summary

• Replace real(real64) with real(wp) and remove redundant local iso_fortran_env imports in raster RGB extraction routines.
• Aligns with repo style and avoids kind mismatches if wp changes.

Changes

• src/backends/raster/fortplot_raster_rendering.f90: use wp in raster_extract_rgb_data
• src/backends/raster/fortplot_raster.f90: use wp in raster_extract_rgb_data_context

Verification
Commands run:

• make build
• make test-ci
• make verify-artifacts

Key outputs:

• ALL TESTS PASSED (fpm test)
• CI essential test suite completed successfully
• Artifact verification passed (examples under output/example/fortran/*)
  • e.g., output/example/fortran/pcolormesh_demo/pcolormesh_basic.pdf contains RGB Image XObject
  • ylabel stripe checks passed for output/example/fortran/ylabel_left/*.png

No functional changes observed; this is a consistency cleanup.

Reviewer updates

• Also aligned wp usage in animation extract API and ASCII backend support.
• Rebuilt, re-ran test-ci and verify-artifacts; all passed locally.