Rewrite shaders to use OpenGL ES3 syntax

Signed-off-by: renderexpert <expert@renderconsulting.co.uk>

libs/shaders/GL/texturing_gui.fsh.glsl

@@ -0,0 +1,23 @@
+layout (location = 0) in vec2 v_colorTexCoords;
+
+layout (location = 0) out vec4 v_FragColor;
+
+layout (binding = 0) uniform UBO
+{
+  mat4 u_modelView;
+  mat4 u_projection;
+  vec2 u_contrastGamma;
+  vec2 u_position;
+  float u_isOutlinePass;
+  float u_opacity;
+  float u_length;
+};
+
+layout (binding = 1) uniform sampler2D u_colorTex;
+
+void main()
+{
+  vec4 finalColor = texture(u_colorTex, v_colorTexCoords);
+  finalColor.a *= u_opacity;
+  v_FragColor = finalColor;
+}