shader: require GPU_shaders5 when gs_num_invocations is given

Fixes a compile error:

   vrend_compile_shader: context error reported 7 "deqp-gles31" Illegal shader 0
   shader failed to compile

   0:3(33): error: GL_ARB_gpu_shader5 invocations qualifier specified

   1: #version 150
   2: #extension GL_ARB_shader_bit_encoding : require
   3: layout(triangles, invocations = 2) in;
   4: layout(points, max_vertices = 3) out;
   ...

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: John Bates <jbates@chromium.org>

src/vrend_shader.c

@@ -1731,6 +1731,7 @@ iter_property(struct tgsi_iterate_context *iter,
       break;
    case TGSI_PROPERTY_GS_INVOCATIONS:
       ctx->gs_num_invocations = prop->u[0].Data;
+      ctx->shader_req_bits |= SHADER_REQ_GPU_SHADER5;
       break;
    case TGSI_PROPERTY_NUM_CLIPDIST_ENABLED:
       ctx->shader_req_bits |= SHADER_REQ_CLIP_DISTANCE;