Add the powerline block arrow symbols to the set of box drawing characters

Fixes #14

This is slightly hackish since those symbols are not defined in the
unicode spec (they are in the private use area) so it is conceivable
that there exist some applications that would break, however, I know of
no such applications, if any show up in the future, a config option to
control this can be added.

kitty/fonts/box_drawing.py

@@ -118,6 +118,31 @@ def cross(*s, a=1, b=1, c=1, d=1):
     half_vline(*s, level=d, which='bottom')
 
 
+def line_equation(x1, y1, x2, y2):
+    m = (y2 - y1) / (x2 - x1)
+    c = y1 - m * x1
+
+    def y(x):
+        return m * x + c
+
+    return y
+
+
+def triangle(buf, width, height, left=True):
+    ay1, by1, y2 = 0, height - 1, height // 2
+    if left:
+        x1, x2 = 0, width - 1
+    else:
+        x1, x2 = width - 1, 0
+    uppery = line_equation(x1, ay1, x2, y2)
+    lowery = line_equation(x1, by1, x2, y2)
+    xlimits = [(uppery(x), lowery(x)) for x in range(width)]
+    for y in range(height):
+        offset = y * width
+        for x, (upper, lower) in zip(range(width), xlimits):
+            buf[x + offset] = 255 if upper <= y <= lower else 0
+
+
 box_chars = {
     '─': [hline],
     '━': [p(hline, level=3)],
@@ -147,6 +172,8 @@ box_chars = {
     '╽': [half_vline, p(half_vline, level=3, which='bottom')],
     '╾': [p(half_hline, level=3), p(half_hline, which='right')],
     '╿': [p(half_vline, level=3), p(half_vline, which='bottom')],
+    '': [triangle],
+    '': [p(triangle, left=False)],
 }
 
 t, f = 1, 3