Fixed GLSL ES compilation error in GLTestAtlasTextRenderer

The test was issuing GL code without precision modifiers in a GLSL
ES context. This wasn't being detected as a fatal error (which is
a separate issue that should probably be addressed) but was
resulting in errors being logged from dm on (at least)
Test-iOS-Clang-iPhone6-GPU-PowerVRGX6450-arm64-Release-All-Metal.

Bug: skia:9266
Change-Id: Ic9b22ed2c66e37ed670270c377364de3bdd15f41
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/243042
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Ethan Nicholas <ethannicholas@google.com>
1 file changed