Auto merge of #3121 - jyavenard:1493198_2, r=gw3583

Fix 10/12 bits rendering on GLES3 (and core GL3+).

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Author: bors-servo

webrender/src/device/gl.rs

@@ -2223,7 +2223,7 @@ impl Device {
                 pixel_type: gl::UNSIGNED_BYTE,
             },
             ImageFormat::R16 => FormatDesc {
-                internal: gl::RED as _,
+                internal: gl::R16 as _,
                 external: gl::RED,
                 pixel_type: gl::UNSIGNED_SHORT,
             },