Fix wrong clipping when rendering curves in Splash

Splash::strokeNarrow may detect clipping of y1 at yMaxI, even if it
shouldn't clip because y1 and yMaxI actually stem from the same
double precission y path value.

That's because y1 is calculated as (int)floor(y), while yMaxI is
calculated as (int)ceil(y) - 1 in SplashClip::clipToRect. Results are
sometimes different, depending on the exact double precission value.

When doing the clipping, x1 may get overly increased, leading to a too
long span in x-direction. The high increment is likely an unrelated second
issue, which I did not investigate here.

Fixes #990, because it avoids the "too much increment" situation in first place.
7 jobs for bugfix/990_path_clipping in 5 minutes and 20 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
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passed #5611921
build

00:03:37

passed #5611926
build_android

00:00:49

passed #5611922
build_clang_libcpp

00:05:19

passed #5611925
build_clazy

00:05:05

passed #5611924
build_mingw64_fedora

00:02:00

passed #5611923
build_ubuntu_16_04

00:02:39

passed #5611920
clang_format

00:00:48