opengl: optimize search for a free slot in texture atlas

Change-Id: Ic853457871b914f9c1beb2f648bf7d9d18dce957
(cherry picked from commit 4823b6d4)
Reviewed-on: https://gerrit.libreoffice.org/18624Reviewed-by: Michael Meeks <michael.meeks@collabora.com>
Tested-by: Michael Meeks <michael.meeks@collabora.com>

vcl/opengl/FixedTextureAtlas.cxx

@@ -36,15 +36,18 @@ OpenGLTexture FixedTextureAtlasManager::InsertBuffer(int nWidth, int nHeight, in
 {
     ImplOpenGLTexture* pTexture = nullptr;
 
-    for (size_t i = 0; i < mpTextures.size(); i++)
+    auto funFreeSlot = [] (std::unique_ptr<ImplOpenGLTexture>& mpTexture)
     {
-        if (mpTextures[i]->mnFreeSlots > 0)
-        {
-            pTexture = mpTextures[i].get();
-        }
-    }
+        return mpTexture->mnFreeSlots > 0;
+    };
+
+    auto aIterator = std::find_if(mpTextures.begin(), mpTextures.end(), funFreeSlot);
 
-    if (!pTexture)
+    if (aIterator != mpTextures.end())
+    {
+        pTexture = (*aIterator).get();
+    }
+    else
     {
         CreateNewTexture();
         pTexture = mpTextures.back().get();