render/shaders: Use high precision for everything

src/backend/render/shaders/offscreen.frag

@@ -6,7 +6,7 @@
 #extension GL_OES_EGL_image_external : require
 #endif
 
-precision mediump float;
+precision highp float;
 #if defined(EXTERNAL)
 uniform samplerExternalOES tex;
 #else

src/backend/render/shaders/rounded_outline.frag

@@ -1,4 +1,4 @@
-precision mediump float;
+precision highp float;
 uniform float alpha;
 #if defined(DEBUG_FLAGS)
 uniform float tint;

src/backend/render/shaders/rounded_rectangle.frag

@@ -1,4 +1,4 @@
-precision mediump float;
+precision highp float;
 uniform float alpha;
 #if defined(DEBUG_FLAGS)
 uniform float tint;