Utilities: BMP save functionality

[DanielFonsecaa]

Implement utilities for saving the current render as a BMP file.

Tasks

• Implement BMP save header writer (writes correct BMP header for 24-bit image)
  • compose header structure
  • write to file
• Capture current frame buffer into a BMP data block (frame buffer converted to BMP pixel layout)
  • reorder BGR
  • handle padding
  • call header writer

Acceptance Criteria

• BMP files are correctly written and readable by standard viewers.