[core] fix text blurriness at different FOVs

ported from -js: c52a09639ceeeb809cd837360993df9c45b45420

This replaces a hardcoded shader approximation with a more correct,
FOV-dependent one.

`gl_Position.w - 0.5`
is replaced with
`gl_Position.w / tan(fov)`

The `/ tan(fov)` is handled by multiplying `1 / tan(fov)` into `u_gamma`
outside the shader.

I think `- 0.5` was just chosen as something that looked right for the
default fov.lease enter the commit message for your changes. Lines starting

src/mbgl/programs/symbol_program.cpp

@@ -75,9 +75,9 @@ static SymbolSDFProgram::UniformValues makeSDFValues(const style::SymbolProperty
     // The default gamma value has to be adjust for the current pixelratio so that we're not
     // drawing blurry font on retina screens.
     const float gammaBase = 0.105 * values.sdfScale / values.paintSize / pixelRatio;
-    const float gammaScale = values.pitchAlignment == AlignmentType::Map
+    const float gammaScale = (values.pitchAlignment == AlignmentType::Map
         ? 1.0 / std::cos(state.getPitch())
-        : 1.0;
+        : 1.0) / state.getAltitude();
 
     return makeValues<SymbolSDFProgram::UniformValues>(
         values,