render/gles2: default to highp if available

The spec [1] says that the maximum value for a mediump float
is at least 2¹⁴ in section 4.5.2. However, when using a 4k
resolution texture coordinates will exceed this value. This causes
issues on drivers which implement mediump as a 16-bit [2].

Switch to highp. There's a twist: on GLES2, support for highp is
optional. So we need to guard it with cute GL_FRAGMENT_PRECISION_HIGH
ifdefs.

[1]: https://registry.khronos.org/OpenGL/specs/es/2.0/GLSL_ES_Specification_1.00.pdf
[2]: https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/21082

(cherry picked from commit 677a3f2f88)

render/gles2/shaders/quad.frag

@@ -1,4 +1,9 @@
+#ifdef GL_FRAGMENT_PRECISION_HIGH
+precision highp float;
+#else
 precision mediump float;
+#endif
+
 varying vec4 v_color;
 varying vec2 v_texcoord;
 uniform vec4 color;

render/gles2/shaders/tex_external.frag

@@ -1,6 +1,11 @@
 #extension GL_OES_EGL_image_external : require
 
+#ifdef GL_FRAGMENT_PRECISION_HIGH
+precision highp float;
+#else
 precision mediump float;
+#endif
+
 varying vec2 v_texcoord;
 uniform samplerExternalOES texture0;
 uniform float alpha;

render/gles2/shaders/tex_rgba.frag

@@ -1,4 +1,9 @@
+#ifdef GL_FRAGMENT_PRECISION_HIGH
+precision highp float;
+#else
 precision mediump float;
+#endif
+
 varying vec2 v_texcoord;
 uniform sampler2D tex;
 uniform float alpha;

render/gles2/shaders/tex_rgbx.frag

@@ -1,4 +1,9 @@
+#ifdef GL_FRAGMENT_PRECISION_HIGH
+precision highp float;
+#else
 precision mediump float;
+#endif
+
 varying vec2 v_texcoord;
 uniform sampler2D tex;
 uniform float alpha;