Optimize simple marker rendering perfomance

expression( "..." ) call create QString object at every invocation. For
layers with simple markers this overhead take upto 10% of rendering time.

Move expression finding to startRender() method.

Signed-off-by: Yauhen Kharuzhy <jekhor@gmail.com>
Eliminate creating of unused QString objects in transformCoords

Every call of transformCoords() Qstring dir object is created. It uses
only for output information about errors, so move its creation to error
handling block.

Perfomance gain is ~7%.

Signed-off-by: Yauhen Kharuzhy <jekhor@gmail.com>
Properly reserve place for arrays in line and polygon clipping routines

A QVector::clear() method completely reinitializes the object. Calling
a QVector::reserve() before clear() is useless and causes big overhead at
array append operations.

Give ~6% perfomance gain.

Signed-off-by: Yauhen Kharuzhy <jekhor@gmail.com>