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.

Shader's parameters are stored in VBO_ITEM. Changed VBO_ITEM data structure. Added UseShader() function for selecting shader for a given VBO_ITEM.
Added one main vertex & fragment shader program to be used for with all kinds of items (type of shader is selected using attributes that are stored in VBO). Currently available shaders are: at-least-1px-width line, filled circle and stroked circle.
Removed unnecessary param (aDepthOffset) from a few functions (OPENGL_GAL::drawSemiCircle(), OPENGL_GAL::drawLineCap()). Removed function OPENGL_GAL::DrawRoundedSegment(). Changed some asserts to debug info or error log.

Display linking errors in debug mode for shaders. Added SHADER::GetAttribute() function for getting shaders attribute location.

Added functionality for transformation of VBO vertices, extended functions like translate, rotate, scale using glm library. Removed D() macro, as it was not used, but in conflict with glm library. Added VBO_ITEMs for circles, semicircles. Now almost everything is drawn using VBO (besides polygons and grid).

Removed BeginLayer and EndLayer functions. Resolved Cairo layers drawing problem in a different way.

Tidied up event handlers.