code: Cleanup and warning fixes from the Vulkan PR (#6163)

Co-authored-by: emufan4568 <geoster3d@gmail.com>
Co-authored-by: Kyle Kienapfel <Docteh@users.noreply.github.com>

src/video_core/renderer_opengl/gl_rasterizer.cpp

@@ -428,6 +428,8 @@ static GLenum GetCurrentPrimitiveMode() {
     default:
         UNREACHABLE();
     }
+
+    return GL_TRIANGLES;
 }
 
 bool RasterizerOpenGL::AccelerateDrawBatchInternal(bool is_indexed) {

src/video_core/renderer_opengl/gl_resource_manager.cpp

@@ -89,7 +89,7 @@ void OGLTexture::CopyFrom(const OGLTexture& other, GLenum target, GLsizei levels
     glActiveTexture(GL_TEXTURE0);
     glBindTexture(GL_TEXTURE_2D, handle);
 
-    for (u32 level = 0; level < levels; level++) {
+    for (GLsizei level = 0; level < levels; level++) {
         glCopyImageSubData(other.handle, target, level, 0, 0, 0, handle, target, level, 0, 0, 0,
                            width >> level, height >> level, 1);
     }

src/video_core/renderer_opengl/renderer_opengl.cpp

@@ -744,7 +744,7 @@ void RendererOpenGL::ReloadShader() {
 void RendererOpenGL::ConfigureFramebufferTexture(TextureInfo& texture,
                                                  const GPU::Regs::FramebufferConfig& framebuffer) {
     GPU::Regs::PixelFormat format = framebuffer.color_format;
-    GLint internal_format;
+    GLint internal_format{};
 
     texture.format = format;
     texture.width = framebuffer.width;

src/video_core/renderer_opengl/texture_downloader_es.cpp

@@ -208,7 +208,7 @@ GLuint TextureDownloaderES::ConvertDepthToColor(GLuint level, GLenum& format, GL
 void TextureDownloaderES::GetTexImage(GLenum target, GLuint level, GLenum format, GLenum type,
                                       GLint height, GLint width, void* pixels) {
     OpenGLState state = OpenGLState::GetCurState();
-    GLuint texture;
+    GLuint texture{};
     const GLuint old_read_buffer = state.draw.read_framebuffer;
     switch (target) {
     case GL_TEXTURE_2D: