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.