Bug 1850815 - Make cs_border_solid's vColorLine highp. r=gfx-reviewers,bradwerth

This varying's precision being reduced to mediump in bug bug 1823411
is causing rendering glitches on certain Mali devices. It should never
have been reduced in the first place, as calculations regarding
positions may require the extra precision. This patch therefore
reverts it to highp.

Differential Revision: https://phabricator.services.mozilla.com/D188984

gfx/wr/webrender/res/cs_border_solid.glsl

@@ -15,7 +15,7 @@ flat varying mediump vec4 vColor1;
 
 // A point + tangent defining the line where the edge
 // transition occurs. Used for corners only.
-flat varying mediump vec4 vColorLine;
+flat varying highp vec4 vColorLine;
 
 // A boolean indicating that we should be mixing between edge colors.
 // Packed in to a vector to work around bug 1630356.