Add RTT (Render-to-Texture) support for RmlUi GridView

This enables 3D model previews for units/features without buildpics
in RmlUi mode, using OpenGL texture rendering.

RTT Implementation:
- Updated _UpdateRmlUiGrid() to handle both file paths and texture IDs
  * String paths: regular image files (adjusted for LuaUI/ prefix)
  * Number IDs: OpenGL textures using ":t{id}" format
  * Table objects: extract texture ID from texID/id fields
- item.SetImage() now triggers grid refresh when texture changes
- item.Invalidate() triggers grid refresh for RTT updates
- Added throttling to prevent excessive DOM updates (max 10 FPS)
  * Schedules delayed update if called too frequently
  * Prevents performance issues from rapid RTT invalidations

The existing RTT rendering code (gl.CreateTexture, gl.RenderToTexture)
works identically in both modes since it's pure OpenGL. The only change
needed was to support texture ID references in RmlUi <img> tags.

Assumes RmlUi supports Spring's ":t<textureID>" format for OpenGL
texture references in image src attributes.

Author: claude

scen_edit/view/grid_view.lua

@@ -262,6 +262,7 @@ function GridView:NewItem(tbl)
         self.layoutPanel:AddChild(item)
     else
         -- RmlUi mode - create simple table item
+        local gridView = self  -- Capture reference to parent grid
         item = {
             name = 'grid_item' .. tostring(__gridItemCounter),
             tooltip = tbl.tooltip,
@@ -271,10 +272,19 @@ function GridView:NewItem(tbl)
         -- Add wrapper method for setting images
         item.SetImage = function(self, imagePath)
             self.image = imagePath
+            -- Trigger grid update when image changes (for RTT updates)
+            if gridView then
+                gridView:_UpdateRmlUiGrid()
+            end
         end
 
         -- Stub methods that might be called
-        item.Invalidate = function() end
+        item.Invalidate = function()
+            -- Trigger grid refresh when item is invalidated (e.g., RTT texture updated)
+            if gridView then
+                gridView:_UpdateRmlUiGrid()
+            end
+        end
         item.IsInView = function() return true end
         item.AddChild = function() end
         item.SetChildLayer = function() end
@@ -403,6 +413,25 @@ function GridView:_UpdateRmlUiGrid()
         return
     end
 
+    -- Throttle updates to avoid excessive DOM manipulation (e.g., from RTT invalidations)
+    -- Maximum 10 updates per second
+    local now = Spring.GetTimer()
+    if self._lastUpdateTime then
+        local elapsed = Spring.DiffTimers(now, self._lastUpdateTime)
+        if elapsed < 0.1 then
+            -- Too soon, schedule delayed update
+            if not self._updateScheduled then
+                self._updateScheduled = true
+                SB.delay(function()
+                    self._updateScheduled = false
+                    self:_UpdateRmlUiGrid()
+                end)
+            end
+            return
+        end
+    end
+    self._lastUpdateTime = now
+
     -- Update the RmlUi DOM to reflect current grid state
     -- This will be called whenever the grid needs to refresh
     if not SB.view or not SB.view.mainDocument then
@@ -427,10 +456,22 @@ function GridView:_UpdateRmlUiGrid()
         -- Add image if present
         if item.image then
             local imagePath = item.image
-            if imagePath:sub(1, 6) == "LuaUI/" then
-                imagePath = "../../../" .. imagePath
+            -- Handle both file paths (strings) and texture IDs (numbers/tables)
+            if type(imagePath) == "string" then
+                -- Regular file path
+                if imagePath:sub(1, 6) == "LuaUI/" then
+                    imagePath = "../../../" .. imagePath
+                end
+                html = html .. string.format('<img src="%s" class="grid-item-image"/>', imagePath)
+            elseif type(imagePath) == "number" then
+                -- OpenGL texture ID - RmlUi should support this directly
+                -- Format: :t<textureID> for Spring texture references
+                html = html .. string.format('<img src=":t%d" class="grid-item-image"/>', imagePath)
+            else
+                -- Texture table/object - try to extract ID or convert to string
+                local texId = imagePath.texID or imagePath.id or tostring(imagePath)
+                html = html .. string.format('<img src=":t%s" class="grid-item-image"/>', tostring(texId))
             end
-            html = html .. string.format('<img src="%s" class="grid-item-image"/>', imagePath)
         end
 
         -- Add caption if present