Add explicit export dimensions to ChartEncoder

[PHJ2000]

Adds explicit width and height to ChartEncoder file, stream and byte-array exports, so callers can choose the output size without rebuilding a chart.

ChartEncoder.saveChart(chart, "chart", "png", 1920, 1080);

The overloads reuse the existing raster/vector/PDF encoders through an IChart view with the requested dimensions. Charts are laid out at that size rather than scaling a rendered image. Invalid dimensions are rejected before opening the output file.

Rendering follows the existing IChart.paint behavior: XChart charts retain the last rendered dimensions. This is documented in the API and README. Existing overloads are unchanged.

Includes TestForIssue807 and tests for XY/pie/category charts, PNG/JPEG dimensions, SVG viewBox and PDF page size, stream ownership, file extensions, and invalid dimensions preserving existing files.

Validation: Java 11 mvn clean verify — 175 tests passed. The demo also rendered at 1280×720 and 600×800 headlessly. The repository skips mvn fmt:check; changed Java files pass a direct check with its google-java-format 1.23.0 version.

Fixes #807.

[timmolter]

Thanks for taking this on, and for the thorough tests. We went a different direction for #807 in #1036, which has now been merged: Chart.setSize(width, height) is a public fluent setter, so a chart received from elsewhere can be resized before calling any of the existing ChartEncoder methods.

chart.setSize(1920, 1080);
ChartEncoder.saveChart(chart, "./MyChart", "png");

That covers the same use case with a single method instead of three overloads, and it makes the size change explicit rather than a side effect of exporting. Closing this one in favor of that approach. Your other two PRs (#1033 and #1035) are still being looked at.