Bumped minimal version of exts related to layout() GLSL qualifier to 1.50.

Related to issue #27 -- layout() qualifier causes compiler errors on
GLSL 1.40 (GL 3.1) on Mac OS X even that the extensions are marked as
supported. Bumping the minimal required version disables the extensions
on this versions, which should avoid the issue.

It is not proper fix at all, but most of the GL3 implementations are
now at least GL 3.2, so it shouldn't be too much of an issue. The only
exception is Mesa (GL 2.1/3.1 on most of the machines), but we need
to maintain GL 2.1 compatibility for it anyway.

src/Extensions.h

@@ -100,7 +100,7 @@ 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, /*!*/ GL310, GL330) // #79
+        _extension(GL,ARB,explicit_attrib_location, /*!*/ GL320, 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
@@ -124,7 +124,7 @@ 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, /*!*/ GL310, GL420) // #108
+        _extension(GL,ARB,shading_language_420pack, /*!*/ GL320, 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
@@ -141,7 +141,7 @@ 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, /*!*/ GL310, GL430) // #128
+        _extension(GL,ARB,explicit_uniform_location, /*!*/ GL320, 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
@@ -268,14 +268,15 @@ namespace GL {
     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.
+    with GLSL 1.20 (compiler error related to layout qualifier on Mesa), 1.30
+    (compiler error related to layout qualifier on NVidia) or 1.40 (compiler
+    error on Mac OS X), bumping minimal required version to GL 3.2 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.
+    with GLSL 1.40 (compiler error related to layout qualifier, similar to the
+    above), bumping minimal required version to GL 3.2 even that it has *3.0*
+    as minimal.
 */
 
 }