app_server: do not return a glyph when nothing is found in the font.

- Fixes BFont::GetHasGlyphs, the "empty square" which was returned led
  it to think the font had glyphs for everything
- This means the "no character" empty square will not be drawn anymore,
  if we want it back, we will need to rework the implementation a bit
  more (either request it explicitly when there is a missing glyph, or
  return it as it was before but including an info that it is the
  "missing glyph")
- Maybe GetHasGlyphs should also bypass the font fallback system, and
  return what's actually in the requested font only.

This also fixes a locking problem in the GlyphLayoutEngine, the code
didn't handle the read/write lock properly and tried to ReadUnlock from
a place where only the write lock was held.

src/servers/app/font/FontCacheEntry.cpp

@@ -304,7 +304,8 @@ FontCacheEntry::CreateGlyph(uint32 glyphCode, FontCacheEntry* fallbackEntry)
 			// get the normal space glyph
 			glyphIndex = engine->GlyphIndexForGlyphCode(0x20 /* space */);
 		} else {
-			// render the "missing glyph box" (by simply keeping glyphIndex 0)
+			// The glyph was not found anywhere.
+			return NULL;
 		}
 	}
 

src/servers/app/font/GlyphLayoutEngine.h

@@ -309,9 +309,15 @@ GlyphLayoutEngine::_WriteLockAndAcquireFallbackEntry(
 	// glyphs from it. We need to obtain the fallback font while we have not
 	// locked anything, since locking the FontManager with the write-lock held
 	// can obvisouly lead to a deadlock.
+	
+	bool writeLocked = entry->IsWriteLocked();
 
-	cacheReference.SetTo(NULL, false);
-	entry->ReadUnlock();
+	if (writeLocked) {
+		entry->WriteUnlock();
+	} else {
+		cacheReference.SetTo(NULL, false);
+		entry->ReadUnlock();
+	}
 
 	if (gFontManager->Lock()) {
 		// TODO: We always get the fallback glyphs from VL Gothic at the
@@ -342,8 +348,10 @@ GlyphLayoutEngine::_WriteLockAndAcquireFallbackEntry(
 		return false;
 	}
 
-	// Update the FontCacheReference, since the locking kind changed.
-	cacheReference.SetTo(entry, true);
+	if (!writeLocked) {
+		// Update the FontCacheReference, since the locking kind changed.
+		cacheReference.SetTo(entry, true);
+	}
 	return true;
 }