Set the version of the BC3 shaders to 310 es

The shaders are compatible with GLSL ES version 310. Require the
GL_ARB_ES3_1_compatibility extension and change the version directives
accordingly so that we get validation from the GL implementation.

bin/Data/bc1.glsl

@@ -1,4 +1,4 @@
-#version 430 core
+#version 310 es
 
 #if defined(GL_ES) && GL_ES == 1
 	// Desktop GLSL allows the const keyword for either compile-time or

bin/Data/bc1_dither.glsl

@@ -1,4 +1,4 @@
-#version 430 core
+#version 310 es
 
 #define BC1_DITHER
 #include "bc1.glsl"

bin/Data/bc4.glsl

@@ -1,4 +1,4 @@
-#version 430 core
+#version 310 es
 
 #if defined(GL_ES) && GL_ES == 1
 	// Desktop GLSL allows the const keyword for either compile-time or

bin/Data/etc2_rgba_stitch.glsl

@@ -2,7 +2,7 @@
 // This compute shader merely stitches them together to form the final result
 // It's also used by RG11 driver to stitch two R11 into one RG11
 
-#version 430 core
+#version 310 es
 
 // #include "/media/matias/Datos/SyntaxHighlightingMisc.h"
 

src/PlatformGL.cpp

@@ -69,12 +69,20 @@ namespace betsy
 
 		g_glContext = SDL_GL_CreateContext( g_sdlWindow );
 
+		const SDL_bool has_required_extensions = SDL_GL_ExtensionSupported( "GL_ARB_ES3_1_compatibility" );
+
 		if( !g_glContext )
 		{
 			fprintf( stderr, "GL Context creation failed.\n" );
 			SDL_Quit();
 			abort();
 		}
+		else if ( has_required_extensions == SDL_FALSE )
+		{
+			fprintf( stderr, "GL Context lacks required extensions.\n" );
+			SDL_Quit();
+			abort();
+		}
 		else
 		{
 			printf( "GL Context creation suceeded.\n" );