🐛 Plotting: fix XCrySDen OpenGL initialisation in VMs

XCrySDen fails to start in VirtualBox VMs with the error "Couldn't configure togl
widget". The root cause is that XCrySDen requests an OpenGL accumulation buffer by
default, which is not supported by virtual GPUs (SVGA3D, llvmpipe). This causes the
togl widget to fail during context creation, even when GLX and Mesa are properly set up.

The fix is to disable the accumulation buffer in XCrySDen's custom-definitions file
by setting `toglOpt(accum)` to false. This is done automatically during provisioning
by copying the default custom-definitions file to `~/.xcrysden/` and patching the
relevant line with `lineinfile`.

Note that this issue does not affect bare-metal installations, where real GPUs support
the accumulation buffer without issue.

Author: mbercx

local/tasks/plotting-tools.yml

@@ -11,6 +11,31 @@
     name: "{{ packages | join(' ') }}"
   register: plot_versions
 
+- name: "Configure XCrySDen for virtual machine compatibility"
+  # XCrySDen requests an OpenGL accumulation buffer by default, which is not
+  # supported by virtual GPUs (SVGA3D, llvmpipe). Disabling it allows the togl
+  # widget to initialize successfully in a VM.
+  when: "'xcrysden' in packages"
+  become: true
+  become_user: "{{ vm_user }}"
+  block:
+  - name: "Create ~/.xcrysden directory"
+    ansible.builtin.file:
+      path: "/home/{{ vm_user }}/.xcrysden"
+      state: directory
+      mode: "0755"
+  - name: "Copy custom-definitions"
+    ansible.builtin.copy:
+      src: /usr/share/xcrysden/Tcl/custom-definitions
+      dest: "/home/{{ vm_user }}/.xcrysden/custom-definitions"
+      remote_src: true
+      mode: "0644"
+  - name: "Disable accumulation buffer"
+    ansible.builtin.lineinfile:
+      path: "/home/{{ vm_user }}/.xcrysden/custom-definitions"
+      regexp: '.*set toglOpt\(accum\).*'
+      line: "set toglOpt(accum) false"
+
 - name: "Document versions"
   loop: "{{ plot_versions.data | dict2items }}"
   loop_control: