Developer manual: Add more instructions for installing development
environment
Adds an appendix with explicit step-by-step instructions on how to make
a development environment for XCsoar from scratch, starting with a fresh
debian install (using Ubuntu as an example).
Forms/List: Smarter "ensureVisible(i)" method, to make sure that the
selected list item is near the center rather than at the very bottom of
the list
box.

ensureVisible(i) now jumps to center the item i when i is more than
about half of the list length outside of the currently visible screen.
In all other cases the behaviour is as before.
New Device: Identify Onyx Boox T68 device and set color and epaper type
flags correctly. Does not yet include dither switch problem workaround
(will be separate patch).
OpenGL/Canvas: Implemented InvertRectangle() methods for OpenGL.

Uses GL_BLEND for the inversion, as glLogicOp(GL_XOR) is not reliable on
all implementations.
Onyx Boox T68 Lynx: Full support for screen dithering. Workaround for
the spurious A2 mode switch problem.

Device now switches to dithering,inverted mode; which is reliable. As a
consequence, the screen has to be inverted, implemented in TopCanvas
just before buffer flipping.
Determine max natively supported line width of OpenGL implementation
Clamp linewidth requested by Pen to maximally available width.
Correct incorrectly reported screen DPI values on some Android devices

Unfortunately, many Android devices report incorrect x and y pixel dpi
values for their screens in the getDisplayMetrics() call. This includes
some popular devices. On affected devices where the error is larger than
30%, we override the reported numbers with correct ones. The list is
quite incomplete at this point.
(Change made in preparation for introducing more extensive line width
scaling)

Background: Usually, in Android the x and y dpi values are rarely used,
instead the "screenDPI" property is used, which has bucketed values of
160, 240, 320
and so on
Map Rendering: Scale line width of topography lines (partially solves
#3267)
Should make ultra-narrow roads and rivers visible on some high pixel
density devices. Only works on devices with an OpenGL implementation
which natively supports variable linewidth .