android: LayerRenderer - use highp and flip in vertex shader

Change-Id: Ia517b0d94fdfb3f8fdd9b9c383c8fb337173932c

android/experimental/LOAndroid3/src/java/org/mozilla/gecko/gfx/LayerRenderer.java

@@ -99,6 +99,11 @@ public class LayerRenderer implements GLSurfaceView.Renderer {
 
     // The shaders run on the GPU directly, the vertex shader is only applying the
     // matrix transform detailed above
+
+    // Note we flip the y-coordinate in the vertex shader from a
+    // coordinate system with (0,0) in the top left to one with (0,0) in
+    // the bottom left.
+
     public static final String DEFAULT_VERTEX_SHADER =
         "uniform mat4 uTMatrix;\n" +
         "attribute vec4 vPosition;\n" +
@@ -106,18 +111,21 @@ public class LayerRenderer implements GLSurfaceView.Renderer {
         "varying vec2 vTexCoord;\n" +
         "void main() {\n" +
         "    gl_Position = uTMatrix * vPosition;\n" +
-        "    vTexCoord = aTexCoord;\n" +
+        "    vTexCoord.x = aTexCoord.x;\n" +
+        "    vTexCoord.y = 1.0 - aTexCoord.y;\n" +
         "}\n";
 
-    // Note we flip the y-coordinate in the fragment shader from a
-    // coordinate system with (0,0) in the top left to one with (0,0) in
-    // the bottom left.
+    // We use highp because the screenshot textures
+    // we use are large and we stretch them alot
+    // so we need all the precision we can get.
+    // Unfortunately, highp is not required by ES 2.0
+    // so on GPU's like Mali we end up getting mediump
     public static final String DEFAULT_FRAGMENT_SHADER =
-        "precision mediump float;\n" +
+        "precision highp float;\n" +
         "varying vec2 vTexCoord;\n" +
         "uniform sampler2D sTexture;\n" +
         "void main() {\n" +
-        "    gl_FragColor = texture2D(sTexture, vec2(vTexCoord.x, 1.0 - vTexCoord.y));\n" +
+        "    gl_FragColor = texture2D(sTexture, vTexCoord);\n" +
         "}\n";
 
     public void setCheckerboardBitmap(Bitmap bitmap, float pageWidth, float pageHeight) {