Add configurable RGB565 byte order support to JPEG encoding

[DT-art1]

Description

This PR adds support for selecting the byte order when converting RGB565 input to RGB888 in the JPEG encoder. Previously, the encoder always assumed big-endian (MSB-first) RGB565 pixel ordering. This matches most ESP32 camera driver outputs, but does not match some custom frame pipelines and external RGB sources, which instead produce little-endian RGB565.

To support both cases, this PR introduces:

void jpgSetRgb565BE(bool enable);

• true → Big-endian RGB565 (default, existing behavior)
• false → Little-endian RGB565

No existing behavior is changed unless the new function is explicitly used.

Changes

• Adds jpgSetRgb565BE(bool) function
• Adds internal rgb565_big_endian state flag (default true)
• Updates convert_line_format() to select byte-order at runtime
• No performance impact.

Related

Testing

• Verified BE → unchanged output.
• Verified LE → correct color output in pipelines producing LE RGB565 frames.

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Checklist

Before submitting a Pull Request, please ensure the following:

• 🚨 This PR does not introduce breaking changes.
• All CI checks (GH Actions) pass.
• Documentation is updated as needed.
• Tests are updated or added as necessary.
• Code is well-commented, especially in complex areas.
• Git history is clean — commits are squashed to the minimum necessary.