make line-opacity work with line-image

js: 0e9a86e3553913abb8ebb3d8926e2b99db8d4008

src/mbgl/renderer/painter_line.cpp

@@ -138,6 +138,7 @@ void Painter::renderLine(LineBucket& bucket, util::ptr<StyleLayer> layer_desc, c
         linepatternShader->u_pattern_tl = imagePos.tl;
         linepatternShader->u_pattern_br = imagePos.br;
         linepatternShader->u_fade = fade;
+        linepatternShader->u_opacity = properties.opacity;
 
         MBGL_CHECK_ERROR(glActiveTexture(GL_TEXTURE0));
         spriteAtlas.bind(true);

src/mbgl/shader/linepattern.fragment.glsl

@@ -6,6 +6,7 @@ uniform vec2 u_pattern_size;
 uniform vec2 u_pattern_tl;
 uniform vec2 u_pattern_br;
 uniform float u_fade;
+uniform float u_opacity;
 
 uniform sampler2D u_image;
 
@@ -31,7 +32,7 @@ void main() {
 
     vec4 color = texture2D(u_image, pos) * (1.0 - u_fade) + u_fade * texture2D(u_image, pos2);
 
-    color.rgb *= color.a; // premultiply
+    alpha *= u_opacity;
 
     gl_FragColor = color * alpha;
 }

src/mbgl/shader/linepattern_shader.hpp

@@ -22,6 +22,7 @@ public:
     Uniform<float>                u_point        = {"u_point",        *this};
     Uniform<float>                u_blur         = {"u_blur",         *this};
     Uniform<float>                u_fade         = {"u_fade",         *this};
+    Uniform<float>                u_opacity      = {"u_opacity",      *this};
 
 private:
     int32_t a_pos = -1;