Capture `EXT_texture_from_pixmap` textures

[yshui]

Description

When using renderdoc with X11 compositors, I find the window textures are all black. X11 compositor uses the EXT_texture_from_pixmap GLX extension to create GL textures from X11 pixmaps. Maybe this is because renderdoc doesn't dump the contents of such textures?

Environment

• RenderDoc version: 1.26
• Operating System: Linux
• Graphics API: OpenGL/GLX