Allocate enough memory for overhang pixels in quarter-rendering of un…

…even columns.

Columns with an uneven width in quarter rendering need a backing buffer for
the overhang pixel for quarter rendering (which is rounded to next even
pixel).

Issues #115

src/unicode-block-canvas.cc

@@ -419,13 +419,14 @@ char *UnicodeBlockCanvas::RequestBuffers(int width, int height) {
  + SCREEN_END_OF_LINE_LEN); // Finishing a line.
 
  // Depending on even/odd situation, we might need one extra row.
- const size_t new_backing = width * (height + 1) * sizeof(rgba_t);
+ // For quarter, we have one extra possible pixel wider.
+ const size_t new_backing = (width + 1) * (height + 1) * sizeof(rgba_t);
  if (new_backing > backing_buffer_size_) {
  backing_buffer_ = (rgba_t *)realloc(backing_buffer_, new_backing);
  backing_buffer_size_ = new_backing;
  }
 
- const size_t new_empty = width * sizeof(rgba_t);
+ const size_t new_empty = (width + 1) * sizeof(rgba_t);
  if (new_empty > empty_line_size_) {
  empty_line_ = (rgba_t *)realloc(empty_line_, new_empty);
  empty_line_size_ = new_empty;