Windows|qmake: Basic configuration for MinGW

Now that Doomsday is based on Qt, support for MinGW is within the
realm of possibility. This commit sets up the basic build rules.
However, a successful build is not yet possible, as many source files
assume WIN32 implies MSVC/_MSC_VER.

Fixed a couple of issues in the Doomsday SDK libs when compiling
with MinGW.
Fixed|Windows|MinGW: Various issues when building with MinGW

Mostly regarding pragmas and compiler/linker options.
Fixed|Windows|MinGW: Various build issues with MinGW

Pragmas and linker options.
Merge branch 'master' into revise-map-geometry-generation
Refactor|Renderer: Began removing render state from geometry generation

When preparing subspace Shards, if the geometry for the middle wall
section fully covers the opening between floor and ceiling then flag
the line segment accordingly. This means one can determine the angle
occlusion for a subspace independently, at render time.

Reporting drawn map lines is now done directly in drawSubspace()

Removed ambient light color state from SectorCluster::prepareShards()

Plus cleanup (still plenty more to do, however).