Reduce LLFolderView per-item memory + fix three folderview defects

View items (cost paid per item, per open inventory window):
- Intern per-item-constant layout + colors into a shared LLFolderViewItemStyle
  const*, deduplicated across items (~100 B/item); also drop two dead fields
  (mLeftPad, mTextPadRight).
- Lazily build the label/suffix LLFontVertexBuffers and release them when an item
  goes off-screen (setVisible override), so only on-screen items hold text
  geometry instead of every built item.

Defects:
- draw(): the icon-overlay branch dereferenced the nullable mIcon for its height;
  fall back to the overlay's own height.
- onIdleUpdateFavorites(): the 'nothing changed' path left mFavoritesDirtyFlags
  set, leaking a per-idle callback that rescanned children every frame; clear it
  and deregister.
- arrange(): drop two redundant sFonts map lookups (use getLabelFont()/sSuffixFont).

Inventory bridge:
- Remove dead shadowed base LLInvFVBridge::mTimeSinceRequestStart (~16 B per
  bridge, per window); only LLFolderBridge's own copy is used.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: RyeMutt

indra/llui/llfolderview.cpp

@@ -207,7 +207,7 @@ LLFolderView::LLFolderView(const Params& p)
     mAutoOpenCandidate = NULL;
     mAutoOpenTimer.stop();
     mKeyboardSelection = false;
-    mIndentation =  getParentFolder() ? getParentFolder()->getIndentation() + mLocalIndentation : 0;
+    mIndentation =  getParentFolder() ? getParentFolder()->getIndentation() + mStyle->localIndentation : 0;
 
     //clear label
     // go ahead and render root folder as usual
@@ -230,11 +230,11 @@ LLFolderView::LLFolderView(const Params& p)
     // Textbox
     LLTextBox::Params text_p;
     LLFontGL* font = getLabelFontForStyle(mLabelStyle);
-    //mIconPad, mTextPad are set in folder_view_item.xml
-    LLRect new_r = LLRect(rect.mLeft + mIconPad,
-                  rect.mTop - mTextPad,
+    //icon_pad, text_pad are set in folder_view_item.xml (shared via mStyle)
+    LLRect new_r = LLRect(rect.mLeft + mStyle->iconPad,
+                  rect.mTop - mStyle->textPad,
                   rect.mRight,
-                  rect.mTop - mTextPad - font->getLineHeight());
+                  rect.mTop - mStyle->textPad - font->getLineHeight());
     text_p.rect(new_r);
     text_p.name(std::string(p.name));
     text_p.font(font);