Removed most of deprecated OpenGL calls. Items used to be drawn in immediate mode now are drawn using vertex arrays.

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.