fix(document): cap popup size at the image's natural size

[szsolt]

Problem

With only_render_image_at_cursor_mode = "popup", the popup window is always sized at
half the screen width, regardless of how large the image actually is. A small image — a
78x20 shields.io badge, say — gets a half-screen float with the image in one corner and a
large empty region beside it.

The aspect-ratio fit can also overshoot vertically on a tall, narrow image, pushing the
float past the bottom of the screen.

Change

Cap the requested width at the image's natural size in cells, so the popup never asks for
more room than the image can fill:

local natural_cols = math.ceil(image.image_width / term_size.cell_width)
local requested_cols = math.min(natural_cols, math.floor(term_size.screen_cols / 2))

Half the screen remains the upper bound, so large images behave exactly as before — only
images narrower than that are affected.

Then clamp the fitted height to the screen:

height = math.min(height, math.max(1, term_size.screen_rows - 4))

Notes

Platform-independent; not specific to any backend or terminal. Verified with badge-sized
and full-width images in a markdown buffer.