Fix underline and strikethrough not being drawn across spaces

[kay-ws]

An underlined run that contains a space is drawn with gaps: ESC[4mA B C
produces three separate underline segments where other terminals produce one.
The same applies to strikethrough.

Cause

Screen::drawText() splits a line into runs of glyphs and runs of spaces.
Glyph runs reach drawGlyph(), which draws the decoration; space runs are
handled by a bare fillRect() that paints the background and nothing else:

if (draw_space) {
    draw_space = false;
    fillRect(startx, y, x - startx, FH(1), bc);
}

There are two such call sites — one inside the loop, one after it — and
neither is passed ul or st.

Fix

Rather than repeating the fillRect() pair at each site, the decoration is
factored into Screen::drawDecoration() and called from all three places. The
y offsets then live in one place, which is what keeps the glyph path and the
space path from drifting apart again — the drift is exactly what this bug is.

drawGlyph() passes its own h, which may have been clipped against the
bottom of the screen, rather than letting the helper re-read FH(1).

Reproducing

printf '\e[4mA B C\e[0m\n'
printf '\e[4m日本語 中文\e[0m\n'

Top: before, three and two segments respectively. Bottom: after, one continuous
underline in both cases, with the double-width cells still underlined for their
full width. Same framebuffer region, only the binary changed.

Verification

Built and tested on x86_64 (Arch, i915drmfb console), where char is signed
and the underline path is otherwise healthy, so the change can be observed on
its own. Also present in an armv7l build; the two captures above are from the
same framebuffer region with only the binary changed.

The change itself has no platform-dependent parts.

[kay-ws]

@k-filipek — you added the decoration rendering in ccf83cf and #1 and #2 both
surfaced with it, so tagging you here rather than opening a separate thread.

There are three PRs open, all small and independent:

• Fix underline and half-bright being discarded where plain char is unsigned #1 — src/lib/vterm.h: SGR 4/SGR 2 discarded where plain char is unsigned
• Fix underline and strikethrough not being drawn across spaces #2 (this one) — src/screen.cpp, src/screen.h: decorations not drawn across spaces
• Handle CSI intermediate bytes instead of printing the final byte #3 — src/lib/vterm.h, src/lib/vterm_states.cpp: CSI intermediate bytes print
  their final byte

All three still merge cleanly. The one with a shelf life is this one:
src/screen.cpp moved in 2eb28a5 (SDL2) after the PR was opened, and
src/screen.h in c1502c0 (DRM/KMS) shortly before it. If it is easier to take
these once that work settles, I am glad to rebase whenever it suits you.
src/lib/vterm.h and vterm_states.cpp have been quiet since bde12cf, so #1
and #3 are in no hurry.

No rush on my side — I would just rather these not turn into conflicts you have
to untangle.