Make Vulkan shader/UBO/layout path real enough for end-to-end blits

- Add shared UBO layout across vertex + fragment stages so vertex
  uniforms like ove_mvpmat compile and bind correctly.
- Rewrite uniform extraction/injection to avoid corrupting the UBO
  block and to keep explicit sampler bindings stable.
- Make the UBO and sampler descriptor bindings visible to both vertex
  and fragment stages.
- Add layout locations for vertex varyings (ove_texcoord) and fragment
  inputs so SPIR-V generation succeeds.
- Extend TransitionImageLayout() to cover all layout pairs used by
  upload/download/clear/blit.
- Add subpass dependencies to the cached render pass and leave
  destinations in SHADER_READ_ONLY_OPTIMAL after Blit().
- Move descriptor allocation before command recording and abort cleanly
  if allocation fails; switch Blit() to the persistent linear sampler.
- Add a gtest that creates a Vulkan backend, uploads a red U8 RGBA
  texture, blits through the default pass-through shader, and verifies
  the downloaded pixel is red. This test passes on the current system.

ctest passes 4/4; DynamicRenderBackend.* passes 3/1 (Vulkan fallback
skipped because Vulkan is available).

app/render/vulkan/vulkanrenderer.h

@@ -115,9 +115,14 @@ private:
 	bool CompileGlslToSpv(const QString &glsl, VkShaderStageFlagBits stage,
 						  QByteArray *out_spv);
 	QString ConvertGlslToVulkan(const QString &glsl, VkShaderStageFlagBits stage);
-	QString ConvertGlslUniformsToUbo(const QString &glsl,
-									 QVector<UniformInfo> *out_uniforms,
-									 int *out_sampler_count = nullptr);
+	QString EnsureGlslVersion450(const QString &glsl) const;
+	void ExtractUniforms(const QString &glsl, QVector<UniformInfo> *out_uniforms,
+						 QVector<QString> *out_samplers) const;
+	void ComputeUniformLayout(QVector<UniformInfo> *uniforms) const;
+	QString BuildUboBlock(const QVector<UniformInfo> &uniforms) const;
+	QString RewriteShaderWithUbo(const QString &glsl,
+							   const QVector<UniformInfo> &all_uniforms,
+							   const QHash<QString, int> &sampler_bindings) const;
 	VkDeviceSize GetStd140Size(const QString &type) const;
 	VkDeviceSize GetStd140Alignment(const QString &type) const;