Splash: Enhance bilinear scaling for downsampling

This is an additional attempt to counter #950.

Splash bilinear scaling was originally limited to up sampling. With small
enhancements we can now use it for down sampling too.

It is useful because the current Splash down sampling algorithm
(weighted average) is notably blurry when scaling down only a bit.
Bilinear is an improvement there, so we use it above 75%.

On the other hand it should never be used for down sampling below 50%,
because bilinear will then sample non-adjacent pixels.
7 jobs for splash_bilinear_downscale in 6 minutes and 15 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Build
failed #4592820
build

00:02:51

passed #4592825
build_android

00:02:04

passed #4592821
build_clang_libcpp

00:06:15

passed #4592824
build_clazy

00:02:49

passed #4592823
build_mingw64_fedora

00:04:11

passed #4592822
build_ubuntu_16_04

00:01:50

passed #4592819
clang_format

00:00:59

 
Name Stage Failure
failed
build Build
Generation complete.
$ wget http://download.qt.io/online/qtsdkrepository/linux_x64/desktop/qt6_600/qt.qt6.600.gcc_64/6.0.0-0-202009070614qtbase-Linux-RHEL_7_6-GCC-Linux-RHEL_7_6-X86_64.7z
--2020-09-18 20:47:39-- http://download.qt.io/online/qtsdkrepository/linux_x64/desktop/qt6_600/qt.qt6.600.gcc_64/6.0.0-0-202009070614qtbase-Linux-RHEL_7_6-GCC-Linux-RHEL_7_6-X86_64.7z
Resolving download.qt.io (download.qt.io)... 77.86.229.90
Connecting to download.qt.io (download.qt.io)|77.86.229.90|:80... connected.
HTTP request sent, awaiting response... 404 Not Found
2020-09-18 20:47:39 ERROR 404: Not Found.

ERROR: Job failed: exit code 1