Implement JPEG output backend with extracted raster graphics functionality

[krystophny]

Summary

Implement JPEG output backend similar to the existing PNG backend, following single responsibility principle by extracting common raster graphics functionality.

Background

Currently we have a PNG backend that works well for animation streaming and static image output. We need to add JPEG support while maintaining clean architecture.

Requirements

1. JPEG Backend Implementation

   • Create fortplot_jpeg.f90 module similar to fortplot_png.f90
   • Support JPEG compression with quality settings
   • Maintain same API pattern as PNG backend (create_jpeg_canvas, get_jpeg_data)

2. Extract Common Raster Functionality (Single Responsibility Principle)

   • Current PNG backend has raster-specific code mixed with PNG-specific code
   • Extract common raster operations into shared module
   • Both PNG and JPEG backends should use shared raster base

3. Architecture Requirements

   • Follow existing pattern in fortplot_animation.f90 for backend-agnostic rendering
   • Support memory-based JPEG generation for animation streaming
   • Maintain compatibility with existing PNG functionality

Proposed Structure

src/fortplot_raster_base.f90    # Common raster operations
src/fortplot_png.f90            # PNG-specific encoding (refactored)
src/fortplot_jpeg.f90           # New JPEG encoding

Reference Implementation

Use STB image library as reference for JPEG encoding:
https://github.com/nothings/stb/blob/master/stb_image.h

STB provides a clean, single-header implementation that can guide our Fortran JPEG encoder.

Implementation Notes

• Use libjpeg or similar for JPEG compression
• Maintain same quality as PNG for scientific visualization
• Follow TDD approach with tests first
• Reference matplotlib JPEG backend for quality standards

Acceptance Criteria

• JPEG backend creates valid JPEG files
• Animation streaming works with JPEG frames
• Common raster functionality extracted to shared module
• All existing PNG functionality remains working
• Tests cover JPEG output quality and correctness

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