Allow shader layout qualifiers only with GLSL 1.40 (GL 3.1) and above.

Even if the extension specification describes the minimum required
version as GL 2.1 (or doesn't mention it at all), most compilers can't
handle `layout(...)` before input/output declaration. These drivers mark
the extensions as supported on GL < 3.1, but then can't handle them at
all: Mesa fails to compile it with GLSL 1.20, NVidia fails with GLSL
1.30.

Also updated `EXPLICIT_*` defines in Shaders/compatibility.glsl to
reflect this change.

src/Extensions.h

@@ -99,7 +99,7 @@ namespace GL {
         _extension(GL,ARB,texture_query_lod,            GL210, GL400) // #73
         _extension(GL,ARB,texture_compression_bptc,     GL310, GL420) // #77
         _extension(GL,ARB,blend_func_extended,          GL210, GL330) // #78
-        _extension(GL,ARB,explicit_attrib_location,     GL210, GL330) // #79
+        _extension(GL,ARB,explicit_attrib_location, /*!*/ GL310, GL330) // #79
         _extension(GL,ARB,occlusion_query2,             GL210, GL330) // #80
         _extension(GL,ARB,sampler_objects,              GL210, GL330) // #81
         _extension(GL,ARB,shader_bit_encoding,    /*?*/ GL210, GL330) // #82
@@ -123,7 +123,7 @@ namespace GL {
         _extension(GL,ARB,viewport_array,               GL210, GL410) // #100
         _extension(GL,ARB,robustness,                   GL210,  None) // #105
         _extension(GL,ARB,base_instance,                GL210, GL420) // #107
-        _extension(GL,ARB,shading_language_420pack,     GL300, GL420) // #108
+        _extension(GL,ARB,shading_language_420pack, /*!*/ GL310, GL420) // #108
         _extension(GL,ARB,transform_feedback_instanced, GL210, GL420) // #109
         _extension(GL,ARB,compressed_texture_pixel_storage, GL210, GL420) // #110
         _extension(GL,ARB,conservative_depth,           GL300, GL420) // #111
@@ -140,7 +140,7 @@ namespace GL {
         _extension(GL,ARB,texture_view,                 GL210, GL430) // #124
         _extension(GL,ARB,vertex_attrib_binding,        GL210, GL430) // #125
         _extension(GL,ARB,ES3_compatibility,            GL330, GL430) // #127
-        _extension(GL,ARB,explicit_uniform_location,    GL210, GL430) // #128
+        _extension(GL,ARB,explicit_uniform_location, /*!*/ GL310, GL430) // #128
         _extension(GL,ARB,fragment_layer_viewport,      GL300, GL430) // #129
         _extension(GL,ARB,framebuffer_no_attachments,   GL210, GL430) // #130
         _extension(GL,ARB,internalformat_query2,        GL210, GL430) // #131
@@ -262,6 +262,20 @@ namespace GL {
 #undef _extension
 #endif
 
+/*
+    Notes (marked with ! above)
+
+    ARB_explicit_attrib_location, ARB_explicit_uniform_location don't work
+    with GLSL 1.20 (compiler error related to layout qualifier on Mesa) or 1.30
+    (compiler error related to layout qualifier on NVidia), bumping minimal
+    required version to GL 3.1 even if both have *2.1* as minimal.
+
+    ARB_shading_language_420pack (particularly sampler bindings) doesn't work
+    with GLSL 1.30 (compiler error related to layout qualifier, similar to the
+    above), bumping minimal required version to GL 3.1 even if it has *3.0* as
+    minimal.
+*/
+
 }
 
 }

src/Shaders/compatibility.glsl

@@ -26,17 +26,21 @@
     #define NEW_GLSL
 #endif
 
+#ifdef GL_ARB_shading_language_420pack
+    #extension GL_ARB_shading_language_420pack: enable
+    #define RUNTIME_CONST
+#endif
+
 /* On NVidia and GLSL 1.20 layout qualifiers result in parsing error, even if
    the extension is defined as supported */
-#if !defined(GL_ES) && __VERSION__ >= 130
+#if !defined(GL_ES) && __VERSION__ >= 140
     #ifdef GL_ARB_explicit_attrib_location
         #extension GL_ARB_explicit_attrib_location: enable
         #define EXPLICIT_ATTRIB_LOCATION
     #endif
-    #if defined(GL_ARB_shading_language_420pack)
+    #ifdef GL_ARB_shading_language_420pack
-        #extension GL_ARB_shading_language_420pack: enable
+        /* Already enabled */
         #define EXPLICIT_TEXTURE_LAYER
-        #define RUNTIME_CONST
     #endif
     #ifdef GL_ARB_explicit_uniform_location
         #extension GL_ARB_explicit_uniform_location: enable