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).

Moved VIEW_ITEM::ViewGetRequiredLayers() functionality to the VIEW class. Now required layers are set per layer basis instead of per item.

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.

Added implementation of level of details per layer and item type pairs basis (items on every layer have a possibility to define the minimum VIEW scale to be shown).

Added the 'cached' parameter for VIEW_LAYER. The parameter decides if items drawn on the layer should be cached or drawn in immediate mode.
Removed m_useGroups from VIEW, as now groups are enabled per layer.

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.

New methods in VIEW class: SetTopLayer(), EnableTopLayer() for managing the top layer display.
New method in PCB_RENDER_SETTINGS class: LoadDisplayOptions() for applying display settings like high-contrast, outline display of items, etc.

New classes:
    - VIEW - represents view that is seen by user, takes care of layer ordering & visibility and how it is displayed (which location, how much zoomed, etc.)
    - VIEW_ITEM - Base class for every item that can be displayed on VIEW (the biggest change is that now it may be necessary to override ViewBBox & ViewGetLayers method for derived classes).
    - EDA_DRAW_PANEL_GAL - Inherits after EDA_DRAW_PANEL, displays VIEW output, right now it is not editable (in opposite to usual EDA_DRAW_PANEL).
    - GAL/OPENGL_GAL/CAIRO_GAL - Base Graphics Abstraction Layer class + two different flavours (Cairo is not fully supported yet), that offers methods to draw primitives using different libraries.
    - WX_VIEW_CONTROLS - Controller for VIEW, handles user events, allows zooming, panning, etc.
    - PAINTER/PCB_PAINTER - Classes that uses GAL interface to draw items (as you may have already guessed - PCB_PAINTER is a class for drawing PCB specific object, PAINTER is an abstract class). Its methods are invoked by VIEW, when an item has to be drawn. To display a new type of item - you need to implement draw(ITEM_TYPE*) method that draws it using GAL methods.
    - STROKE_FONT - Implements stroke font drawing using GAL methods.
                          
Most important changes to Kicad original code:
    * EDA_ITEM now inherits from VIEW_ITEM, which is a base class for all drawable objects.
    * EDA_DRAW_FRAME contains both usual EDA_DRAW_PANEL and new EDA_DRAW_PANEL_GAL, that can be switched anytime.
    * There are some new layers for displaying multilayer pads, vias & pads holes (these are not shown yet on the right sidebar in pcbnew)
    * Display order of layers is different than in previous versions (if you are curious - you may check m_galLayerOrder@pcbnew/basepcbframe.cpp). Preserving usual order would result in not very natural display, such as showing silkscreen texts on the bottom.
    * Introduced new hotkey (Alt+F12) and new menu option (View->Switch canvas) for switching canvas during runtime.
    * Some of classes (mostly derived from BOARD_ITEM) now includes ViewBBox & ViewGetLayers methods.
    * Removed tools/class_painter.h, as now it is extended and included in source code.
                         
Build changes:
    * GAL-based rendering option is turned on by a new compilation CMake option KICAD_GAL.
    * When compiling with CMake option KICAD_GAL=ON, GLEW and Cairo libraries are required.
    * GAL-related code is compiled into a static library (common/libgal).
    * Build with KICAD_GAL=OFF should not need any new libraries and should come out as a standard version of Kicad
                            
Currently most of items in pcbnew can be displayed using OpenGL (to be done are DIMENSIONS and MARKERS).
More details about GAL can be found in: http://www.ohwr.org/attachments/1884/view-spec.pdf

Added template RTREE and class VIEW_RTREE (R-tree for fast spatial indexing of VIEW items; for future usage in GAL)