fake negative x/y viewport positions under vulkan

Since Vulkan doesn't support negative x or y viewport positions, this
patch concatenates a translation matrix to the projection matrix used in
Vulkan rendering. In this way, if a negative x or y coordinate would
be necessary, we can instead use a 0 coordinate and a
leftwards/upwards translation to achieve the same effect.  This seems
to work alright with overlays, so as far as I can tell this patch
seems sufficient to get the behavior we want?

gfx/common/vulkan_common.h

@@ -577,6 +577,8 @@ typedef struct vk
    VkViewport vk_vp;
    VkRenderPass render_pass;
    struct video_viewport vp;
+   float translate_x;
+   float translate_y;
    struct vk_per_frame swapchain[VULKAN_MAX_SWAPCHAIN_IMAGES];
    struct vk_image backbuffers[VULKAN_MAX_SWAPCHAIN_IMAGES];
    struct vk_texture default_texture;

gfx/drivers/vulkan.c

@@ -3411,6 +3411,8 @@ static void *vulkan_init(const video_info_t *video,
    video_driver_get_size(&temp_width, &temp_height);
    vk->video_width       = temp_width;
    vk->video_height      = temp_height;
+   vk->translate_x       = 0.0;
+   vk->translate_y       = 0.0;
 
    RARCH_LOG("[Vulkan]: Using resolution %ux%u.\n", temp_width, temp_height);
 
@@ -3762,17 +3764,30 @@ static void vulkan_set_projection(vk_t *vk,
          0.0f,     0.0f,    0.0f,    0.0f ,
          0.0f,     0.0f,    0.0f,    1.0f }
    };
+   math_matrix_4x4 trn     = {
+      {  1.0f,     0.0f,    0.0f,    0.0f ,
+         0.0f,     1.0f,    0.0f,    0.0f ,
+         0.0f,     0.0f,    1.0f,    0.0f ,
+         vk->translate_x/(float)vk->vp.width,
+         vk->translate_y/(float)vk->vp.height,
+         0.0f,
+         1.0f }
+   };
+   math_matrix_4x4 tmp     = {
+      {  1.0f,     0.0f,    0.0f,    0.0f ,
+         0.0f,     1.0f,    0.0f,    0.0f ,
+         0.0f,     0.0f,    1.0f,    0.0f ,
+         0.0f,     0.0f,    0.0f,    1.0f }
+   };
 
    /* Calculate projection. */
    matrix_4x4_ortho(vk->mvp_no_rot, ortho->left, ortho->right,
          ortho->bottom, ortho->top, ortho->znear, ortho->zfar);
 
    if (!allow_rotate)
+      tmp = vk->mvp_no_rot;
+   else
    {
-      vk->mvp = vk->mvp_no_rot;
-      return;
-   }
-
       radians                 = M_PI * vk->rotation / 180.0f;
       cosine                  = cosf(radians);
       sine                    = sinf(radians);
@@ -3780,7 +3795,9 @@ static void vulkan_set_projection(vk_t *vk,
       MAT_ELEM_4X4(rot, 0, 1) = -sine;
       MAT_ELEM_4X4(rot, 1, 0) = sine;
       MAT_ELEM_4X4(rot, 1, 1) = cosine;
-   matrix_4x4_multiply(vk->mvp, rot, vk->mvp_no_rot);
+      matrix_4x4_multiply(tmp, rot, vk->mvp_no_rot);
+   }
+   matrix_4x4_multiply(vk->mvp, trn, tmp);
 }
 
 static void vulkan_set_rotation(void *data, unsigned rotation)
@@ -3827,8 +3844,6 @@ static void vulkan_set_viewport(void *data, unsigned viewport_width,
             video_driver_get_aspect_ratio(),
             vk->flags & VK_FLAG_KEEP_ASPECT,
             true);
-      vk->vp.x = MAX(vk->vp.x, 0);
-      vk->vp.y = MAX(vk->vp.y, 0);
       viewport_width  = vk->vp.width;
       viewport_height = vk->vp.height;
    }
@@ -3846,6 +3861,21 @@ static void vulkan_set_viewport(void *data, unsigned viewport_width,
       vk->vp.height = viewport_height;
    }
 
+   if (vk->vp.x < 0)
+   {
+      vk->translate_x = (float)vk->vp.x;
+      vk->vp.x = 0.0;
+   }
+   else
+      vk->translate_x = 0.0;
+   if (vk->vp.y < 0)
+   {
+      vk->translate_y = (float)vk->vp.y;
+      vk->vp.y = 0.0;
+   }
+   else
+      vk->translate_y = 0.0;
+
    vulkan_set_projection(vk, &ortho, allow_rotate);
 
    /* Set last backbuffer viewport. */