vrend: Don't crash when no feat_dual_src_blend

When there is no feat_dual_src_blend, avoid calling glBindFragDataLocationIndexed.
Otherwise epoxy could crash the whole process when this function is not available.
Signed-off-by: Lepton Wu <lepton@chromium.org>
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>

src/virgl_hw.h

@@ -555,7 +555,8 @@ enum virgl_ctx_errors {
         VIRGL_ERROR_GL_ANY_SAMPLES_PASSED,
         VIRGL_ERROR_CTX_ILLEGAL_FORMAT,
         VIRGL_ERROR_CTX_ILLEGAL_SAMPLER_VIEW_TARGET,
-        VIRGL_ERROR_CTX_TRANSFER_IOV_BOUNDS
+        VIRGL_ERROR_CTX_TRANSFER_IOV_BOUNDS,
+        VIRGL_ERROR_CTX_ILLEGAL_DUAL_SRC_BLEND
 };
 
 #define VIRGL_RESOURCE_Y_0_TOP (1 << 0)

src/vrend_renderer.c

@@ -1525,12 +1525,18 @@ static struct vrend_linked_shader_program *add_shader_program(struct vrend_conte
 
    if (fs->sel->sinfo.num_outputs > 1) {
       if (util_blend_state_is_dual(&ctx->sub->blend_state, 0)) {
-         glBindFragDataLocationIndexed(prog_id, 0, 0, "fsout_c0");
-         glBindFragDataLocationIndexed(prog_id, 0, 1, "fsout_c1");
+         if (has_feature(feat_dual_src_blend)) {
+            glBindFragDataLocationIndexed(prog_id, 0, 0, "fsout_c0");
+            glBindFragDataLocationIndexed(prog_id, 0, 1, "fsout_c1");
+         } else {
+            report_context_error(ctx, VIRGL_ERROR_CTX_ILLEGAL_DUAL_SRC_BLEND, 0);
+         }
          sprog->dual_src_linked = true;
       } else {
-         glBindFragDataLocationIndexed(prog_id, 0, 0, "fsout_c0");
-         glBindFragDataLocationIndexed(prog_id, 1, 0, "fsout_c1");
+         if (has_feature(feat_dual_src_blend)) {
+            glBindFragDataLocationIndexed(prog_id, 0, 0, "fsout_c0");
+            glBindFragDataLocationIndexed(prog_id, 1, 0, "fsout_c1");
+         }
          sprog->dual_src_linked = false;
       }
    } else