Number of X colormap entries may be less than 1 << depth

[Boris Gjenero]

If I run scrot 1.4-1 to take a screenshot from a MicroVAX with a QDSS display device running ULTRIX 4.5, I get:
X Error of failed request: BadValue (integer parameter out of range for operation)
  Major opcode of failed request: 91 (X_QueryColors)
  Value in failed request: 0xff
  Serial number of failed request: 23
  Current serial number in output stream: 23

The QDSS is an 8 bit PseudoColor device, but it only has 254 available colormap entries, not 256. Here is a fragment from xdpyinfo:
  visual:
    visual id: 0x23
    class: PseudoColor
    depth: 8 planes
    available colormap entries: 254
    red, green, blue masks: 0x0, 0x0, 0x0
    significant bits in color specification: 8 bits

The problem is that in __imlib_GrabDrawableToRGBA(), imlib2 ignores the number of colormap entries and instead does XQueryColors() for all 256 entries, based on the 8 bit depth. The hardware does actually have 256 entries, but the last two are reserved for the hardware cursor, and so the X server denies this XQueryColors() request. The attached patch instead only queries the number of colormap entries based on the visual, and sets the rest of the entries to zero.

ProblemType: Bug
DistroRelease: Ubuntu 20.10
Package: libimlib2 1.6.1-2
ProcVersionSignature: Ubuntu 5.8.0-25.26-generic 5.8.14
Uname: Linux 5.8.0-25-generic x86_64
NonfreeKernelModules: nvidia_modeset nvidia
ApportVersion: 2.20.11-0ubuntu50
Architecture: amd64
CasperMD5CheckResult: skip
CurrentDesktop: XFCE
Date: Tue Nov 10 10:04:44 2020
SourcePackage: imlib2
UpgradeStatus: Upgraded to groovy on 2020-10-22 (19 days ago)

[Ubuntu Foundations Team Bug Bot (crichton)]

The attachment "Fix for this issue" seems to be a patch. If it isn't, please remove the "patch" flag from the attachment, remove the "patch" tag, and if you are a member of the ~ubuntu-reviewers, unsubscribe the team.

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