vo_gpu: vulkan: correctly enable textureGatherOffset

This also requires a vulkan feature / SPIR-V capability to function

video/out/vulkan/ra_vk.c

@@ -222,8 +222,8 @@ struct ra *ra_create_vk(struct mpvk_ctx *vk, struct mp_log *log)
     // UBO support is required
     ra->caps |= RA_CAP_BUF_RO | RA_CAP_FRAGCOORD;
 
-    // textureGather is only supported in GLSL 400+
-    if (ra->glsl_version >= 400)
+    // textureGather requires the ImageGatherExtended capability
+    if (vk->features.shaderImageGatherExtended)
         ra->caps |= RA_CAP_GATHER;
 
     // Try creating a shader storage buffer

video/out/vulkan/utils.c

@@ -502,6 +502,7 @@ bool mpvk_device_init(struct mpvk_ctx *vk, struct mpvk_device_opts opts)
     // Enable all features we optionally use
 #define FEATURE(name) .name = vk->features.name
     VkPhysicalDeviceFeatures feats = {
+        FEATURE(shaderImageGatherExtended),
         FEATURE(shaderStorageImageExtendedFormats),
     };
 #undef FEATURE