Add compatibility with old POS printers + add example

[marcteys]

Summary

Add ESP32 example for downloading and printing BMP images from URLs. This new example combines WiFi/HTTPS downloading with a full BMP parser to enable direct printing of web-hosted images on thermal printers.

Changes

New Example: download_bmp_print

• Complete ESP32 sketch demonstrating BMP download and printing

Key Features

• Full BMP Parser: Supports multiple BMP formats (1-bit, 4-bit, 8-bit paletted, 24-bit, 32-bit)
• Color Conversion: Automatic color-to-monochrome conversion (threshold-based)
• Memory Efficient: Streaming architecture processes images without loading entire file
• Format Handling: Supports both top-down and bottom-up BMP storage formats
• Auto-sizing: Automatically limits image width to printer capability (384 pixels)
• Proper Color Mapping: Bit 1 = black (print), Bit 0 = white (no print)

Technical Implementation

BMP Parsing Functions:

• read16(), read32() - Read BMP header values with proper byte ordering
• skip() - Efficiently skip bytes in stream with timeout protection
• read8n() - Buffered pixel data reading with connection monitoring
• downloadAndPrintBMP() - Main function orchestrating download, parse, and print

Architecture:

• Streams BMP data directly from HTTPS response
• Converts pixels to monochrome bitmap on-the-fly
• Uses configurable input buffer (800 bytes) for efficient memory usage
• Handles color palettes for indexed color BMPs
• Respects BMP row padding (4-byte alignment)

Documentation Updates

• Added new section to README: "Download and print BMP images from URL"
• Included feature list, requirements, and usage examples
• Added to table of contents

Hardware Requirements

• ESP32 board (uses WiFi, HTTPClient, WiFiClientSecure libraries)
• Internet connection
• Thermal printer compatible with TPrinter library

Notes

• Example uses printBitmapOld() function (GS v 0 ESC/POS command)
• WiFi credentials need to be updated before use