Make dot and dash line shaders more precise

The number-of-pieces approach the old implementation had lead to too much
accumulated rounding error, causing periodic truncated dashes. This
lead to lines falling out of their periodic pattern, breaking the clipping
tricks gecko applies downstream.

webrender/res/ps_line.glsl

@@ -59,30 +59,22 @@ void main(void) {
             break;
         }
         case LINE_STYLE_DASHED: {
-            // y = dash on + off length
-            // z = dash length
-            // w = center line of edge cross-axis (for dots only)
-            float desired_dash_length = size.y * 3.0;
-            // Consider half total length since there is an equal on/off for each dash.
-            float dash_count = 1.0 + ceil(size.x / desired_dash_length);
-            float dash_length = size.x / dash_count;
-            vParams = vec4(2.0 * dash_length,
-                           dash_length,
+            float dash_length = size.y * 3.0;
+            vParams = vec4(2.0 * dash_length, // period
+                           dash_length,       // dash length
                            0.0,
                            0.0);
             break;
         }
         case LINE_STYLE_DOTTED: {
             float diameter = size.y;
-            float radius = 0.5 * diameter;
-            float dot_count = ceil(0.5 * size.x / diameter);
-            float empty_space = size.x - dot_count * diameter;
-            float distance_between_centers = diameter + empty_space / dot_count;
+            float period = diameter * 2.0;
             float center_line = pos.y + 0.5 * size.y;
-            vParams = vec4(distance_between_centers,
-                           radius,
+            float max_x = floor(size.x / period) * period; 
+            vParams = vec4(period,
+                           diameter / 2.0, // radius
                            center_line,
-                           0.0);
+                           max_x);
             break;
         }
         case LINE_STYLE_WAVY: {
@@ -223,6 +215,8 @@ void main(void) {
             vec2 dot_relative_pos = vec2(x, pos.y) - vParams.yz;
             float dot_distance = length(dot_relative_pos) - vParams.y;
             alpha = min(alpha, distance_aa(aa_range, dot_distance));
+            // Clip off partial dots
+            alpha *= step(pos.x - vLocalOrigin.x, vParams.w);
             break;
         }
         case LINE_STYLE_WAVY: {