libsysprof-analyze: use doubles for reference data

We only want to move to float for the internal 0..1 positions which are
used to draw to the screen (to reduce how much data we hold on to). But for
the data we need to calculate those 0..1 positions, we want better
precision for large numbers. Use double for all of those.

src/libsysprof-gtk/sysprof-depth-layer.c

@@ -47,7 +47,7 @@ sysprof_depth_layer_snapshot (GtkWidget   *widget,
   SysprofDepthLayer *self = (SysprofDepthLayer *)widget;
   const SysprofXYSeriesValue *values;
   guint n_values;
-  float min_x, max_x;
+  double min_x, max_x;
   int line_width;
   int width;
   int height;