console.lua: deduplicate y position calculation in update()

This deduplicates the calculation of the y position by assigning it to a
y variable, which will also be used in the next commit. margin_x is also
renamed to x for consistency.

player/lua/console.lua

@@ -547,8 +547,8 @@ local function update()
 
     local osd_w, osd_h = get_scaled_osd_dimensions()
 
-    local margin_x = get_margin_x()
-    local margin_y = get_margin_y()
+    local x = get_margin_x()
+    local y = osd_h * (1 - global_margins.b) - get_margin_y()
 
     local coordinate_top = math.floor(global_margins.t * osd_h + 0.5)
     local clipping_coordinates = '0,' .. coordinate_top .. ',' ..
@@ -589,7 +589,7 @@ local function update()
         -- libass/ass_render.c:ass_render_event() subtracts --osd-margin-x from
         -- the maximum text width twice.
         local width_max = math.floor(
-            (osd_w - margin_x - mp.get_property_native('osd-margin-x') * 2 / scale_factor())
+            (osd_w - x - mp.get_property_native('osd-margin-x') * 2 / scale_factor())
             / opts.font_size * get_font_hw_ratio())
 
         local suggestions, rows = format_table(suggestion_buffer, width_max, max_lines)
@@ -608,7 +608,7 @@ local function update()
 
     ass:new_event()
     ass:an(1)
-    ass:pos(margin_x, osd_h - margin_y - global_margins.b * osd_h)
+    ass:pos(x, y)
     ass:append(log_ass .. '\\N')
     ass:append(suggestion_ass)
     ass:append(style .. ass_escape(prompt) .. ' ' .. before_cur)
@@ -619,7 +619,7 @@ local function update()
     -- cursor appear in front of the text.
     ass:new_event()
     ass:an(1)
-    ass:pos(margin_x, osd_h - margin_y - global_margins.b * osd_h)
+    ass:pos(x, y)
     ass:append(style .. '{\\alpha&HFF&}' .. ass_escape(prompt) .. ' ' .. before_cur)
     ass:append(cglyph)
     ass:append(style .. '{\\alpha&HFF&}' .. after_cur)