Added flipping mode.

Smarter way of the overlay rendering (overlay is always refreshed, while cached&noncached targets only if the viewport or items have changed).

Fixed blending function for OpenGL compositing. Corrected documentation, removed unnecessary functions.

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 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).

Added VBO_CONTAINER as a faster storage for vertices (OPENGL_GAL), tuned for exchanging data with GPU.

Removed a few unnecessary variables and fields from OPENGL_GAL.
Added function GAL::ClearCache() for freeing memory used by cached items.
Fixed a few memory leaks (tesselator, PAINTER's settings & VIEW_ITEM's groups).
Changed a few functions into inlines.

Removed indices storing from VBO_ITEM as they are always consecutive numbers. Removed storing pointers to VBO_ITEMs that have to be drawn - instead they are memcpied to mapped GPU memory.
Some functions of VBO_ITEM became inline.