RFR: 8280861: Robot color picker broken on Linux with scaling above 100% [v4]

[Maxim Kartashev]

> The primary API for image capture on modern Linuxes is `gdk_pixbuf_get_from_window()` that expects both coordinates and the size unscaled (i.e. not multiplied by the current desktop scale). At the same time, `gtk3_interface.c:gtk3_get_drawable_data()` gets the coordinates from `Robot` that pre-scales them (multiplying by the scale known to Java, however, not necessarily the current desktop scale). The problem with the size had been partly taken care of in [JDK-8225118](https://bugs.openjdk.java.net/browse/JDK-8225118), but coordinates are still passed to `gdk_pixbuf_get_from_window()` pre-scaled.
> 
> The idea of the fix is to capture a possibly larger area that is guaranteed to contain the one that is interesting to the caller and then only copy the interesting pixels to the output image. As a positive side effect, the size of the captured area cannot be less than 1x1 (provided the correct input, of course). This solves the problem of zero size passed to  `(*fp_gdk_pixbuf_get_from_drawable)` when the desktop scale is 3 and we're asked to capture just one pixel. In that case, the previous formula (`width / (float) win_scale + 0.5`) would have yielded `0`. 
> 
> A related issue would be that tests written specifically for this general area (`java/awt/Robot`) didn't catch this problem and only somewhat unrelated tests (`javax/swing/...`) were affected.
> 
> This one is solved by adding pixel-sized areas to the test image in `HiDPIRobotScreenCaptureTest.java`, which precise colors are then verified to match the expected ones. In addition to that, instead of verifying just the color of the center of a large area, 4 more pixels close to the area's border are checked. This helps to make sure that captured area coordinates aren't way off.
> 
> The tests under `javax/swing/...` that originally helped to identify this problem were modified to provide more screen space for painting their components as extreme desktop scaling (300%) on Ubuntu doesn't leave much room due to a humongous title bar.
> 
> The fix was tested by running all the modified tests on Ubuntu with desktop scaling set to 100%, 200%, and 300%. The `Robot` tests were also executed on Windows (300% scaling) and MacOS.

Maxim Kartashev has updated the pull request incrementally with one additional commit since the last revision:

  Renamed scaling functions and updated copyright.

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/7425/files
  - new: https://git.openjdk.java.net/jdk/pull/7425/files/fc651add..4a8c20af

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=7425&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=7425&range=02-03

  Stats: 8 lines in 3 files changed: 0 ins; 0 del; 8 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7425.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7425/head:pull/7425

PR: https://git.openjdk.java.net/jdk/pull/7425