Optimize QgsVectorLayerFeatureIterator a bit

11% speedup when rendering 1 million points from memory provider
Optimize simple marker: do not construct/destruct "size" all the time

13% speedup when rendering 1m points from memory provider
Optimize simple line drawing when without data-defined properties

20% speed improvement when drawing 1m linestrings (4points each) from memory layer
Skip some work if no data-defined properties are applied

8% speedup with 1m polygons in memory layer