Consistent wp usage in raster RGB extraction (remove real64)

[krystophny]

Align raster RGB extraction with wp kind alias

Problem

• Some raster routines use real(real64) and local use iso_fortran_env, only: real64 instead of the project-preferred wp kind alias already imported at the module level.
• This is inconsistent with repository guidelines and the rest of the raster code, and risks accidental kind mismatches if wp is ever changed.

Scope

• Replace real(real64) with real(wp) in:
  • src/backends/raster/fortplot_raster_rendering.f90 subroutine raster_extract_rgb_data
  • src/backends/raster/fortplot_raster.f90 subroutine raster_extract_rgb_data_context
• Remove redundant inner use iso_fortran_env, only: real64 in those routines.
• Use _wp literals for numeric constants.

Tests/Verification

• Build and run full fast test suite: make test
• Run artifact checks: make verify-artifacts
• No functional change expected; purely consistency/cleanup with style guide.