test: verify raster title centering matches computed pixel position

Author: krystophny

test/test_title_centering_raster.f90

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+program test_title_centering_raster
+    !! Verifies raster title centering over plot area
+    use fortplot_layout, only: plot_margins_t, plot_area_t, calculate_plot_area
+    use fortplot_constants, only: TITLE_VERTICAL_OFFSET
+    use fortplot_text, only: calculate_text_width
+    use fortplot_raster_axes, only: compute_title_position
+    use, intrinsic :: iso_fortran_env, only: wp => real64
+    implicit none
+
+    type(plot_margins_t) :: margins
+    type(plot_area_t) :: plot_area
+    integer, parameter :: CANVAS_WIDTH = 640
+    integer, parameter :: CANVAS_HEIGHT = 480
+
+    character(len=*), parameter :: title_text = 'Simple Sine Wave'
+    character(len=500) :: processed_text, escaped_text
+    integer :: processed_len
+    real(wp) :: title_px_r, title_py_r
+    integer :: expected_px, expected_py, measured_width
+
+    margins = plot_margins_t()
+    call calculate_plot_area(CANVAS_WIDTH, CANVAS_HEIGHT, margins, plot_area)
+
+    call compute_title_position(plot_area, title_text, processed_text, processed_len, &
+                                escaped_text, title_px_r, title_py_r)
+
+    measured_width = calculate_text_width(trim(escaped_text))
+    expected_px = plot_area%left + plot_area%width/2 - measured_width/2
+    expected_py = plot_area%bottom - TITLE_VERTICAL_OFFSET
+
+    if (int(title_px_r) /= expected_px) then
+        print *, 'FAIL: Title X not centered; got ', int(title_px_r), ' expected ', expected_px
+        stop 1
+    end if
+
+    if (int(title_py_r) /= expected_py) then
+        print *, 'FAIL: Title Y offset incorrect; got ', int(title_py_r), ' expected ', expected_py
+        stop 1
+    end if
+
+    print *, 'PASS: Title centering matches expected pixel position'
+end program test_title_centering_raster
+