Increase QOpenGLTextureCache max size

From 64 to 256 MB. Note that this is especially important
because bindTexture() will fail and return an invalid
texture for image data larger than this. That is not ideal
but is trickier to correct. Until that is available, increase
the cache size to alleviate the pain somewhat.

Task-number: QTBUG-59207
Change-Id: Ibc22524acad0b42a632eb7e4cd8ea86225174837
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>

src/gui/opengl/qopengltexturecache.cpp

@@ -97,7 +97,7 @@ void QOpenGLTextureCacheWrapper::cleanupTexturesForPixmapData(QPlatformPixmap *p
 
 QOpenGLTextureCache::QOpenGLTextureCache(QOpenGLContext *ctx)
     : QOpenGLSharedResource(ctx->shareGroup())
-    , m_cache(64 * 1024) // 64 MB cache
+    , m_cache(256 * 1024) // 256 MB cache
 {
 }