Improve glrenderer crash safety

[zackslash]

Memory-safety fixes for the kitty graphics upload path and PTY reader thread, all reachable from PTY output:

• Key kitty texture staleness on the ghostty generation stamp, not byte length (was rendering stale on re-upload).
• Set GL_UNPACK_ALIGNMENT=1 for GL_RGB uploads (was over-reading rows when width%4≠0).
• Clamp image dims to GL_MAX_TEXTURE_SIZE + validate pixelsLen (closes a 32-bit ARM size_t overflow → heap over-read).
• Detach PtyReaderThread in async stop() (was aborting on destruction).