prevent bleeding in screen texture

fixes #920

src/GPU_OpenGL_shaders.h

@@ -22,6 +22,7 @@
 const char* kCompositorVS = R"(#version 140
 
 in vec2 vPosition;
+in vec2 vTexcoord;
 
 smooth out vec2 fTexcoord;
 
@@ -33,7 +34,7 @@ void main()
     fpos.w = 1.0;
 
     gl_Position = fpos;
-    fTexcoord = (vPosition + vec2(1.0, 1.0)) * (vec2(256.0, 384.0) / 2.0);
+    fTexcoord = vTexcoord;
 }
 )";