Issue #1354 - Don't allow glsl[130,400] unless we have gpu_shader5

- Teach GLContext about gpu_shader5
- Downgrade shader language version if gpu_shader5 support isn't found.

gfx/gl/GLContext.h

@@ -102,6 +102,7 @@ enum class GLFeature {
     get_query_object_iv,
     get_string_indexed,
     gpu_shader4,
+    gpu_shader5,
     instanced_arrays,
     instanced_non_arrays,
     internalformat_query,
@@ -421,6 +422,7 @@ public:
         ARB_framebuffer_object,
         ARB_framebuffer_sRGB,
         ARB_geometry_shader4,
+        ARB_gpu_shader5,
         ARB_half_float_pixel,
         ARB_instanced_arrays,
         ARB_internalformat_query,
@@ -460,6 +462,7 @@ public:
         EXT_framebuffer_object,
         EXT_framebuffer_sRGB,
         EXT_gpu_shader4,
+        EXT_gpu_shader5,
         EXT_multisampled_render_to_texture,
         EXT_occlusion_query_boolean,
         EXT_packed_depth_stencil,
@@ -486,6 +489,7 @@ public:
         NV_fence,
         NV_framebuffer_blit,
         NV_geometry_program4,
+        NV_gpu_shader5,
         NV_half_float,
         NV_instanced_arrays,
         NV_primitive_restart,