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.

Added separate layers for pad netnames (now these are divided into multilayer/top/bottom pads).
More appropriate layers are selecting a copper layer in the high contrast mode (now it shows the copper layer itself, vias & multilayer pads and netnames).

1 pixel wide outlines for the Cairo and the OpenGL with shaders backends, but it breaks displaying outline in the shaderless OpenGL backend.

Refactored code responsible for high contrast mode. Now it allows to have more than one layer on the top.
Selecting layer using the dropdown list on the toolbar influences the layer displayed in high contrast mode.

Scripting engine build fix.

Eeschema: fix a *very subtle* bug noticeable only in complex hierachies, for multiple parts per package:
sometimes, the modified flag was set for these components just when switching from a sheet to an other sheet.
Pcbnew: fix Bug #1197414 (dragged track segments does not show clearance area)