Use member variables instead of getResolution(), getScale() and getUnits() on the map

[ahocevar]

Based on #1591, it occurred to me that especially map.getResolution(), which is called on every vector feature being rendered, could be avoided if we had a member variable like currentResolution on the map object. This would have to be update only when zoom or base layer changes. The same is true for getScale(), although this is not used that frequently. And the recently added method getUnits() (#1591) can be avoided at all if map.units is updated in the map.setBaseLayer method.

[crschmidt]

Not touched since 2.7, moving forward.

[fredj]

No patch, bumping.