Make gl_Position(-1, -1) map to texel (0, 0) of the render target

There was a lot of missing around with viewports and flip the Y
coordinate in vertex shaders before. Turns out things are simpler than
we thought: *all* APIs have gl_Position(-1, -1) map to texel (0, 0). It
is just the present coordinate system that changes.

Remove some of the hacks we had to work around non-existent viewport
issues and fix tests.

src/backend/d3d12/ShaderModuleD3D12.cpp

@@ -23,7 +23,6 @@ namespace backend { namespace d3d12 {
         spirv_cross::CompilerHLSL compiler(builder->AcquireSpirv());
 
         spirv_cross::CompilerGLSL::Options options_glsl;
-        options_glsl.vertex.flip_vert_y = false;
         options_glsl.vertex.fixup_clipspace = true;
         compiler.spirv_cross::CompilerGLSL::set_options(options_glsl);
 

src/backend/opengl/ShaderModuleGL.cpp

@@ -56,7 +56,6 @@ namespace backend { namespace opengl {
 #else
         options.version = 440;
 #endif
-        options.vertex.flip_vert_y = true;
         compiler.set_options(options);
 
         // Rename the push constant block to be prefixed with the shader stage type so that uniform