Rendering improvements

[AnonymouX47]

• Changes image resampling method from BICUBIC to BOX.
• Improves transparent renders in text-based styles.
• Implements workaround for un-rendered BG colors with 'block' render style on the kitty terminal.
• Implements auto background color (using the terminal's default BG color for transparent images)
• Optimizes image render data computation and image formatting.
• Adds tests for BaseImage._get_render_data().