OpenGL: tweak legacy 2D shader

EXT_gpu_shader4 lets us say “noperspective” in GLSL #version 120 just
like in later GLSL.

Mac shader now matches modern GLSL available on other platforms.

source/blender/gpu/shaders/gpu_shader_2D_smooth_color_frag.glsl

@@ -1,6 +1,6 @@
 
 #if __VERSION__ == 120
-  varying vec4 finalColor;
+  noperspective varying vec4 finalColor;
   #define fragColor gl_FragColor
 #else
   noperspective in vec4 finalColor;

source/blender/gpu/shaders/gpu_shader_2D_smooth_color_vert.glsl

@@ -3,7 +3,7 @@
   attribute vec2 pos;
   attribute vec4 color;
 
-  varying vec4 finalColor;
+  noperspective varying vec4 finalColor;
 #else
   in vec2 pos;
   in vec4 color;