Added implementation of level of details per layer and item type pairs basis (items on every layer have a possibility to define the minimum VIEW scale to be shown).

Added the 'cached' parameter for VIEW_LAYER. The parameter decides if items drawn on the layer should be cached or drawn in immediate mode.
Removed m_useGroups from VIEW, as now groups are enabled per layer.

Fixed drawing of lines' caps.

Fixed drawing stroked semicircles using OpenGL backend.

VBO_CONTAINER: Changed new[]/delete[] pairs to realloc(), possibly reducing memory fragmentation and the container shrinking time.

Faster circles & semicircles drawing for the shaderless OpenGL backend. Removed unnecessary variables and computations.