ossia binding: file view sized from bytes read; average-color example covers its input

[jcelerier]

Two fixes needed by the score graphics branch (ossia/score#2303).

• ossia binding: size a file view from the bytes read. A text-mode read drops every carriage return, so sizing the view from the file's size on disk ran it past the end of the buffer. The view now covers the bytes actually read.
• examples: the average-color compute example averages its whole input. It wrote with a fixed stride of 16 past its buffer, covered at most 256×256 pixels, and divided by the block count instead of the pixel count. The score-side test test_gfx_average_color_fixg pins the fixed behaviour.

🤖 Generated with Claude Code

https://claude.ai/code/session_01JeL19ofVgPe9u9q7J1V9ex