Make z-order changes work on web too, restoring tooltips

widgetry/src/shaders/fragment_300.glsl

@@ -5,7 +5,7 @@ precision mediump sampler2DArray;
 
 // (x offset, y offset, zoom)
 uniform vec3 transform;
-// (window width, window height, _)
+// (window width, window height, z value)
 uniform vec3 window;
 
 in vec4 pass_style;

widgetry/src/shaders/vertex_300.glsl

@@ -7,20 +7,33 @@ uniform vec3 transform;
 // (window width, window height, z value)
 uniform vec3 window;
 
-layout (location = 0) in vec2 position;
+layout (location = 0) in vec3 position;
 layout (location = 1) in vec4 style;
 out vec4 pass_style;
 
 void main() {
     pass_style = style;
 
+    float zoom = transform[2];
+
     // This is map_to_screen
-    float screen_x = (position[0] * transform[2]) - transform[0];
-    float screen_y = (position[1] * transform[2]) - transform[1];
-    // Translate that to clip-space or whatever it's called
+    float screen_x = (position[0] * zoom) - transform[0];
+    float screen_y = (position[1] * zoom) - transform[1];
+
+    // Translate position to normalized device coordinates (NDC)
     float x = (screen_x / window[0] * 2.0) - 1.0;
     float y = (screen_y / window[1] * 2.0) - 1.0;
 
+    // largest absolute value for layer base z values in drawing.rs (i.e.
+    // MAPSPACE_Z vs. TOOLTIP_Z)
+    float z_range = 2.0;
+    // we increment z up to 1.0 to affect ordering within a layer, so consider
+    // that when scaling z to NDC too
+    float z_scale = z_range + 1.0;
+
+    float z = (position[2] + window[2]) / z_scale;
+
+
     // Note the y inversion
-    gl_Position = vec4(x, -y, window[2], 1.0);
+    gl_Position = vec4(x, -y, z, 1.0);
 }