[core] fix rendering of dashed lines with round caps

src/mbgl/geometry/line_atlas.cpp

@@ -1,5 +1,6 @@
 #include <mbgl/geometry/line_atlas.hpp>
 #include <mbgl/gfx/upload_pass.hpp>
+#include <mbgl/math/log2.hpp>
 #include <mbgl/math/minmax.hpp>
 #include <mbgl/util/hash.hpp>
 #include <mbgl/util/logging.hpp>
@@ -107,8 +108,25 @@ DashPatternTexture::DashPatternTexture(const std::vector<float>& from_,
                                        const LinePatternCap cap) {
     const bool patternsIdentical = from_ == to_;
     const int32_t patternHeight = cap == LinePatternCap::Round ? 15 : 1;
-
-    AlphaImage image({256, static_cast<uint32_t>((patternsIdentical ? 1 : 2) * patternHeight)});
+    const uint32_t height = (patternsIdentical ? 1 : 2) * patternHeight;
+
+    // The OpenGL ES 2.0 spec, section 3.8.2 states:
+    //
+    //     Calling a sampler from a fragment shader will return (R,G,B,A) = (0,0,0,1) if any of the
+    //     following conditions are true:
+    //     […]
+    //     - A two-dimensional sampler is called, the corresponding texture image is a
+    //       non-power-of-two image […], and either the texture wrap mode is not CLAMP_TO_EDGE, or
+    //       the minification filter is neither NEAREST nor LINEAR.
+    //     […]
+    //
+    // This means that texture lookups won't work for NPOT textures unless they use GL_CLAMP_TO_EDGE.
+    // We're using GL_CLAMP_TO_EDGE for the vertical direction, but GL_REPEAT for the horizontal
+    // direction, which means that we need a power-of-two texture for our line dash patterns to work
+    // on OpenGL ES 2.0 conforming implementations. Fortunately, this just means changing the height
+    // from 15 to 16, and from 30 to 32, so we don't waste many pixels.
+    const uint32_t textureHeight = 1 << util::ceil_log2(height);
+    AlphaImage image({256, textureHeight});
 
     from = addDashPattern(image, 0, from_, cap);
     to = patternsIdentical ? from : addDashPattern(image, patternHeight, to_, cap);