BLF: simplify text shader texel_fetch

Avoid very costly integer division/modulo as well as two texelFetch
calls separated by a branch. We know that font texture width
is power of two, so we can replace division/modulo with a shift
and a mask, that is set from the calling code via glyph_tex_size
uniform.

Analyzing the shader with Mali Offline Compiler for Mali-G76 arch:
- Stack spilling: 724 -> 692 bytes
- Total cycles, arithmetic: 119.5 -> 33.2
- Total cycles, load/store: 161.0 -> 119.0
- Total cycles, texture: 88.0 -> 44.0

@@ -15,3 +18,1 @@
-    return texelFetch(glyph, ivec2(index % size_x, index / size_x), 0).r;
-  }
-  return texelFetch(glyph, ivec2(index, 0), 0).r;
+  /* glyph_tex_size: upper 8 bits is log2 of texture width, lower 24 bits is width-1 */

@@ -18,0 +18,4 @@
+  /* glyph_tex_size: upper 8 bits is log2 of texture width, lower 24 bits is width-1 */
+  int col_mask = glyph_tex_size & 0xFFFFFF;
+  int row_shift = glyph_tex_size >> 24;
+  ivec2 uv = ivec2(index & col_mask, index >> row_shift);

@@ -27,3 +32,1 @@
-  vec2 texel_2d = uv * vec2(glyph_dim) + vec2(0.5);
-  ivec2 texel_2d_near = ivec2(texel_2d) - 1;
-  int frag_offset = glyph_offset + texel_2d_near.y * glyph_dim.x + texel_2d_near.x;
+  ivec2 iuv = ivec2(floor(uv)) - 1;

@@ -118,0 +117,4 @@
+        for (int ix = 0; ix < 4; ++ix) {
+          int ofsx = ix - 1;
+          float v = texel_fetch(frag_offset + ofsy * glyph_dim.x + ofsx);
+          if (!is_inside_box(iuv + ivec2(ofsx, ofsy)))

@@ -161,0 +168,4 @@
+          ++idx;
+        }
+      }
+      fragColor.a = sum * (1.0 / 80.0);