Render: add "OpenGL" render engine.

This is intended for quick renders for previsualization, animation previews
or sequencer previews. It provides the same settings as found in the 3D view
Shading popover in solid display mode, but in the scene render properties.

The "Workbench" engine was removed, and this name no longer appears in the
user interface, it's purely an internal name. We might come up with a better
name for this OpenGL engine still, but it's good to be consistent with the
OpenGL Render operator name since this has a similar purpose.

release/scripts/startup/bl_ui/properties_data_metaball.py

@@ -71,7 +71,7 @@ class DATA_PT_metaball(DataButtonsPanel, Panel):
 class DATA_PT_mball_texture_space(DataButtonsPanel, Panel):
     bl_label = "Texture Space"
     bl_options = {'DEFAULT_CLOSED'}
-    COMPAT_ENGINES = {'BLENDER_RENDER', 'BLENDER_EEVEE'}
+    COMPAT_ENGINES = {'BLENDER_RENDER', 'BLENDER_EEVEE', 'BLENDER_OPENGL'}
 
     def draw(self, context):
         layout = self.layout
@@ -125,7 +125,7 @@ class DATA_PT_metaball_element(DataButtonsPanel, Panel):
 
 
 class DATA_PT_custom_props_metaball(DataButtonsPanel, PropertyPanel, Panel):
-    COMPAT_ENGINES = {'BLENDER_RENDER', 'BLENDER_EEVEE'}
+    COMPAT_ENGINES = {'BLENDER_RENDER', 'BLENDER_EEVEE', 'BLENDER_OPENGL'}
     _context_path = "object.data"
     _property_type = bpy.types.MetaBall