fix color bleeding

data/shaders/stroke-pad.glsl

@@ -0,0 +1,37 @@
+[[vertex]]
+
+in vec4 pos;
+out vec2 uv;
+
+void main()
+{
+    gl_Position = pos;
+    uv = pos.xy * 0.5 + 0.5;
+}
+
+[[fragment]]
+
+#include "include/ext-fb-fetch.glsl"
+
+uniform sampler2D tex_bg;
+uniform mediump vec4 col;
+
+in highp vec2 uv;
+#if defined(GL_EXT_shader_framebuffer_fetch)
+    inout highp vec4 frag;
+#else
+    out highp vec4 frag;
+#endif
+
+void main()
+{
+    // sample from the background
+#if defined(GL_EXT_shader_framebuffer_fetch)
+    highp vec4 bg = frag;
+#elif defined(GL_ARM_shader_framebuffer_fetch)
+    highp vec4 bg = gl_LastFragColorARM;
+#else
+    highp vec4 bg = texture(tex_bg, uv);
+#endif
+    frag = bg.a == 0.0 ? vec4(col.rgb, 0.0) : bg;
+}