annotation_demo: Missing annotations in PNG, rendering issues in PDF

[krystophny]

Problem Description

The annotation_demo example has rendering issues in both PNG and PDF backends:

PNG Backend Issues:

• No annotations visible - annotations are completely missing from PNG output
• Only the plot lines and axes are rendered

PDF Backend Issues:

• Strange character rendering - some text appears as "?" characters
• Incorrect positioning - some annotations are placed in wrong locations
• Partial rendering compared to intended output

Expected Behavior

All annotations should render correctly with:

• Proper text display (no "?" characters)
• Correct positioning for all coordinate types (COORD_DATA, COORD_FIGURE, COORD_AXIS)
• Consistent rendering across PNG and PDF backends

Evidence

PNG Output: file:///home/ert/code/fortplot/output/example/fortran/annotation_demo/annotation_demo.png

• Annotations completely missing

PDF Output: file:///home/ert/code/fortplot/output/example/fortran/annotation_demo/annotation_demo.pdf

• Some annotations visible but with rendering issues
• Character encoding problems ("?" instead of proper characters)
• Positioning inconsistencies

Files Involved

• Example: example/fortran/annotation_demo/annotation_demo.f90
• PNG rendering: src/backends/raster/ (annotation rendering)
• PDF rendering: src/backends/vector/fortplot_pdf.f90, src/text/fortplot_annotation_rendering.f90

Impact

• Users cannot rely on annotation functionality
• Examples do not demonstrate working features
• Documentation/tutorial value compromised

Related

This is a separate issue from #1413 which was focused on PDF rendering artifacts (contours, streamlines, mathtext). The annotation coordinate transformation in #1413 fixed some PDF positioning but did not address PNG backend or character encoding issues.