Code refactorization. VBO_CONTAINER is split to [NON]CACHED_MANAGER, GPU_MANAGER and VERTEX_MANAGER.

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

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.

- VBO_CONTAINER::allocate() was returning wrong value in case of error
- framelimiter had wrong formula for computing destined period between frames
- removed _padding field from VBO_VERTEX, as it was not speeding up, but wasting memory

Moved fields containing information about currently used color, shader and transformation for vertices from VBO_ITEM to VBO_CONTAINER (OPENGL_GAL).