Reset smooth scroll timer only if waking up the animation loop

[nihiL7331]

This PR fixes smooth scroll issues at high scrolling speeds. The issue in logic was that ctx->term.last_scroll_ms was updated unconditionally on every frame where it was scrolled, instead of on every frame where it was scrolled AND its the first frame of scrolling (on ctx->term.is_scrolling = 0 => ctx->term.is_scrolling = 1 switch).
This later caused the line:

float dt = (float)(now - ctx->term.last_scroll_ms);

to become 0, after which

float decay = dt * SFTE_TERM_SCROLL_DELAY;

became 0, causing the scroll y offset to not get updated in this line:

ctx->term.scroll_y_offset -= ctx->term.scroll_y_offset * decay

It's fixed by adding a simple if (!ctx->term.is_scrolling) check on the last_scroll_ms update.