tentative fix for gaps in fat line strokes

[mmalex]

trying to fix the bug shown in this tweet:
https://twitter.com/mmalex/status/626162676318781440

works a bit grossly by forcing one short edge of each quad to copy the previous line's edge, provided there isnt too big an angle change. could be made more efficient by sharing vertices that are actually shared... I might do that in a bit.

[mmalex]

I've added sharing of edges (fixes a FIXME) in the second commit, however to avoid looping twice over the edges it does a PrimReserve(0, -number) (second param is negative!) to 'unreserve' a number of verts based on how many shared edges there were. not sure how you feel about that. works here ;)

[ocornut]

Thanks Alex! I'll keep that on hold for a bit - just merged in thick line for the AA-path. It is merging normals in a way that looks pretty awesome: (that's code from mikko mononen)

Compare to your version with a threshold (non-AA path):

So most likely I'll go and fix the non-AA path so that it behave like the AA one. Unless there is a reason for using that technique for some shapes.

PrimReserve with negative values is safe, added a comment. Hasn't occured I could use that (AddText does a similar thing manually). I am currently moving large chunk of the ImDrawList code around, when I'm done I'll look back to make sure the non-AA path behave the same.

Test code

ImDrawList* draw_list = ImGui::GetWindowDrawList();
ImVec2 pos = ImGui::GetCursorScreenPos();

draw_list->PathArcTo(ImVec2(pos.x+100.0f,pos.y+20), 90.0f, 0.0f, 3.1416f);
draw_list->PathArcTo(ImVec2(pos.x+100.0f,pos.y+20), 70.0f, 3.1416f, 0.0f);
draw_list->PathStroke(0xFF00FFFF, true, 12.0f);

pos.y += 150.0f;
draw_list->PathLineTo(ImVec2(pos.x+0,pos.y+0));
draw_list->PathLineTo(ImVec2(pos.x+30,pos.y+100));
draw_list->PathLineTo(ImVec2(pos.x+50,pos.y+10));
draw_list->PathLineTo(ImVec2(pos.x+100,pos.y+90));
draw_list->PathLineTo(ImVec2(pos.x+120,pos.y+0));
draw_list->PathStroke(0xFF00FFFF, false, 12.0f);