separate u_opacity from u_color

This prepares the shaders for the switch to data driven styling where
opacity and color need to be passed to the shader independent of
eachotehr.

remove the `additionalOpacity` parameter from `util.premultiply`

fill.fragment.glsl

@@ -1,7 +1,8 @@
 precision mediump float;
 
 uniform lowp vec4 u_color;
+uniform lowp float u_opacity;
 
 void main() {
-    gl_FragColor = u_color;
+    gl_FragColor = u_color * u_opacity;
 }

line.fragment.glsl

@@ -2,6 +2,7 @@ precision mediump float;
 
 uniform vec2 u_linewidth;
 uniform lowp vec4 u_color;
+uniform lowp float u_opacity;
 uniform float u_blur;
 
 varying vec2 v_normal;
@@ -18,5 +19,5 @@ void main() {
     float blur = u_blur * v_gamma_scale;
     float alpha = clamp(min(dist - (u_linewidth.t - blur), u_linewidth.s - dist) / blur, 0.0, 1.0);
 
-    gl_FragColor = u_color * alpha;
+    gl_FragColor = u_color * (alpha * u_opacity);
 }

linesdfpattern.fragment.glsl

@@ -2,6 +2,7 @@ precision mediump float;
 
 uniform vec2 u_linewidth;
 uniform lowp vec4 u_color;
+uniform lowp float u_opacity;
 uniform float u_blur;
 uniform sampler2D u_image;
 uniform float u_sdfgamma;
@@ -27,5 +28,5 @@ void main() {
     float sdfdist = mix(sdfdist_a, sdfdist_b, u_mix);
     alpha *= smoothstep(0.5 - u_sdfgamma, 0.5 + u_sdfgamma, sdfdist);
 
-    gl_FragColor = u_color * alpha;
+    gl_FragColor = u_color * (alpha * u_opacity);
 }

outline.fragment.glsl

@@ -1,11 +1,12 @@
 precision mediump float;
 
 uniform lowp vec4 u_color;
+uniform lowp float u_opacity;
 
 varying vec2 v_pos;
 
 void main() {
     float dist = length(v_pos - gl_FragCoord.xy);
     float alpha = smoothstep(1.0, 0.0, dist);
-    gl_FragColor = u_color * alpha;
+    gl_FragColor = u_color * (alpha * u_opacity);
 }

sdf.fragment.glsl

@@ -3,6 +3,7 @@ precision mediump float;
 uniform sampler2D u_texture;
 uniform sampler2D u_fadetexture;
 uniform lowp vec4 u_color;
+uniform lowp float u_opacity;
 uniform lowp float u_buffer;
 uniform lowp float u_gamma;
 
@@ -15,5 +16,5 @@ void main() {
     lowp float fade_alpha = texture2D(u_fadetexture, v_fade_tex).a;
     lowp float gamma = u_gamma * v_gamma_scale;
     lowp float alpha = smoothstep(u_buffer - gamma, u_buffer + gamma, dist) * fade_alpha;
-    gl_FragColor = u_color * alpha;
+    gl_FragColor = u_color * (alpha * u_opacity);
 }